There are some big, cosmic no-no’s in D&D. Many are impossible or very difficult for a mortal to accomplish (e.g. gods who don’t belong there¹ walking the Material Plane in the flesh can do a lot of damage and piss absolutely everyone else off, but a mortal isn’t a god and can’t do that), but we can find some.
This is necessarily going to extend beyond the lore of D&D 5e, because there isn’t all that much of that and as far as I’m aware, none of it gets into these things. I know nothing of Pathfinder’s lore.
Messing with the Far Realm
The “Far Realm” refers to any place outside the multiverse. It’s not known if this is a plane, or many planes, or even many multiverses, but whatever is out there does not get along with what’s here. Everything operates under different rules and the reality of each is anathema to the reality of the other. Some aberrations have their origins in the Far Realm, and have somehow adapted to life in our reality, and maybe some from our reality have made something for themselves in that one, but the overwhelming majority of beings from either side of the veil cannot comprehend the other, and cannot survive in an environment that operates on entirely different rules.
For the most part, Far Realm incursions are random, small, and brief. The multiverse itself acts against them, preventing them from being larger than that. But sometimes they are larger, and sometimes they’re larger because someone is making it happen. This is a serious threat to the multiverse—a large enough overlap between ours and the Far Realm could destroy both. So this wouldn’t just piss off our entire multiverse, but multiple!
For my money, this is your best choice for reproducing a reaction like Slughorn’s in the quote: getting involved with the Far Realm is an incomprehensibly bad choice. There’s power there, sort of, but you’re almost certainly not going to get any of it by messing with incursions. In the Complete Arcane supplement for D&D 3.5e, there is an alienist prestige class² that’s all about the Far Realm, especially trying to summon creatures from there. The class literally drives you insane as you take levels of it. That’s kind of how things go with the Far Realm: the risks and drawbacks are so extreme that even if you can get power that way, anyone who tries is going to be regarded as dangerously out of their mind. And since they’re dangerous not only to themselves, but potentially to existence itself, people are likely to react to that.
Messing with history
There are some “sanctioned” time-manipulation powers out there, whether it’s haste or time stop, or even the odd time hop or powerful time regression.³ All of these are extremely limited in terms of how much they actually manipulate time itself, and only time regression allows for manipulating the past—and that only as a 9th-level power and only seconds into the past.
But in AD&D there was a supplement called Chronomancer. It’s a very weird book; it details various magics for more serious interactions with time and introduces “Temporal Prime,” a kind of “plane” from which you can travel through time (sort of like how teleportation involves travel through the Astral). At the same time, it basically said you should never, ever use those powers, and that trying would get you in a lot of trouble with a lot of the multiverse very quickly. You could—and canonical characters did—visit Temporal Prime, but basically the only “allowed” reason to go there was to prevent anyone else from actually messing with history.
Unlike messing with the Far Realm, it’s trivial to understand why someone would want to rewrite history. They’re still gonna stop you, though.
Messing with pre-incarnate souls
The Bastion of Unborn Souls is absolutely off-limits to all and sundry—messing with that is going to get a lot of incredibly powerful folks from across the alignment spectrum extremely angry with you, and they will do something about it. Basically, the gods are barred from messing with souls before they’re in a body, as a protection for mortals’ free will, and if they’re not allowed to do so, they’re going to make damn sure no one else gets to, either.
Ashardalon, a red dragon who has replaced his own heart with that of a balor, who had very-nearly-deific power, did siphon power away from the Bastion in the Bastion of Broken Souls module. He even planned to destroy them all, which would seriously threaten the overall multiverse. But he didn’t try to meddle with individual unborn souls, allowing him to avoid most of the biggest reactions. He was nevertheless hunted down and slain, and in death found no afterlife—he instead became a vestige, a strange manifestation that doesn’t actually exist, in some sense, or exists in some space outside the multiverse.⁴ (Some fans refer to it as the “Near Realm,” to contrast with the truly alien Far Realm well outside the multiverse.)
But if you go in there and start to try taking some of those souls for yourself, rather than just sucking down some of the power of the place? You’re in for a world of hurt, and basically no one will back you up no matter what you were trying to accomplish. But again, there’s lots of good reasons to want to do so, so it wouldn’t get Slughorn’s reaction.
Messing with the Demiplane of Imprisonment
The Demiplane of Imprisonment is where Tharizdun is bound. The Chained God wants nothing less than the complete and total annihilation of all that is, and he seems to have the power to do it.⁵ That means just about everything else that is, is opposed to him getting out. The powers that be, from across the alignment spectrum, had to band together to chain him in the first place, so they’re no doubt ready to do so again to keep him that way.
If you go messing with the Demiplane of Imprisonment, the most likely result is that you’re going to find yourself sucked into it, and then you’re in the prison with Tharizdun—not ideal (though this suboptimal situation will almost certainly be brief). But if you actually have any chance unchaining the Chained One, that will set off alarm bells all across the multiverse. Even Evil isn’t generally in favor of the complete and utter destruction that Tharizdun is; if nothing else, Tharizdun absolutely means to include that evil among the destroyed.
This is probably your second-best bet at something like Slughorn’s reaction, because there’s nothing to be gained by freeing Tharizdun. He won’t thank you; he’ll destroy you like he will everything else. But it’s plausible for someone to believe otherwise; people have.
Defying the Lady of Pain
The Lady of Pain is, in a sense, the touchstone upon which the entire multiverse depends, in that the Spire is the Wheel’s axle and Sigil is the city that resides at the top of the (infinitely tall) Spire, and the Lady is absolute within Sigil. The Lady of Pain is not a goddess, is not a prince or archduke or whatever, she is just the Lady. What she says, goes.
For instance, the Lady of Pain has decreed that gods are not allowed to enter Sigil—and so they don’t. Sigil is the most valuable strategic property in the entire multiverse (being in the middle and having portals within the city that go literally everywhere else), so having even the tiniest foothold there is immensely tempting to literally everyone, but the Lady has said gods aren’t allowed and out of respect for that, gods don’t enter and for that matter, just to give her the space she has requested, neither do their most powerful minions or others of similar stature. It’s not, per se, that they couldn’t, so much as the fact that if they did, the Lady would end them and it wouldn’t even be a contest.
However, Vecna (a lich and the god of secrets, but to my mind better named the god of cheaters), cheated the system in Die, Vecna, Die!. He abused a loophole to become a god while entering Sigil, creating a weird situation where he was a god in Sigil but hadn’t entered Sigil as a god, per se. His divine power within the City of Doors was doing immense damage, and the Lady was concerned that putting her own power against his would result in exacerbating that damage and literally destroy the multiverse. She therefore got some adventurers to get rid of Vecna, which they canonically did. (The adventure notes that if the players fail, the Lady’s next move was to invite a party of demigod adventurers, who would be allowed in on this one special occasion. Bending her own rule was already kind of problematic, though, which is why the Lady went with mortal adventurers first.)
In the process of getting booted out of Sigil, Vecna lost almost all of the divine power he’d cheated his way into, and also there was irreparable harm done to the multiverse: Die, Vecna, Die! is the official in-setting explanation for why the 2e rules changed to the 3e rules. The Lady has made absolutely sure that Vecna’s loopholes have been firmly closed, and no one of note has tested her since.
Considering how much damage Vecna did with this stunt, the Lady would have no shortage of those ready, willing, and able to assist her with any attempt at anything like a repeat performance. Not that she necessarily needs it—Vecna was going to leave Sigil, one way or the other, and nothing he could have done could have stopped it—but as this episode shows, it may be convenient for the Lady to enlist assistance. Again, the entire alignment spectrum would be on board with providing that.
The big things that get everyone’s attention are the most notable answers to this question, just because evil—particularly chaotic evil—really doesn’t care what you do unless it’s going to affect them. The above just affect everyone, and so everyone is on the same page about stopping them.
Whatever becoming a lich entails
Becoming a lich involves some unspecified act of overwhelming evil. It doesn’t bother the Lower Planes per se—they love that evil—but it doesn’t really help the Lower Planes much, either, since it delays, potentially for a considerable length of time, their acquisition of an evil soul. (Not that Evil as a whole is impatient.) But whatever it is, it’s so evil that it taints your soul to the point that you basically cannot die.
I consider it reasonably likely that Rowling’s horcruxes were based on the D&D lich’s phylactery, at least indirectly (that is, she may not have been familiar with D&D or D&D’s lich, but the concept has found its way into a fair bit of fantasy writing over the years since, almost always associated with a “lich” or undead wizard). There are older folk tales with similar themes that Rowling may have been familiar with (“The Giant with No Heart in His Body,” “The Death of Koschei the Deathless”), but in my opinion, the specifics of horcruxes and Voldemort seem closer to D&D liches than to these.
Crimes with Inevitable Sentencing
The inevitables are robots built out of solid Law that hunt down and punish those who commit very specific crimes. I happen to know that in Pathfinder, these are a considerably bigger deal than they are in D&D, since modrons—the Lawful Neutral exemplars of Mechanus—are D&D “Product Identity” and thus off-limits to Paizo, so inevitables replaced them in Pathfinder.
Anyway, in D&D 3e,⁶ from Manual of the Planes (and later reprinted/updated in the 3.5e Monster Manual) we have
- Kolyarut, who punish oath-breakers
- Marut, who punish those who cheat death
- Zelekhut, who punish those who flee justice
and from Fiend Folio we have
- Varakhut, who punish mortals who attempt to ascend to godhood, as well as those would attempt to kill a god
- Quarut, who punish those who use magic to alter reality too hard—a major part of the anti-chronomancy brigade
(There’s also the utterly insipid Anhydrut from Sandstorm, but you—everyone—should ignore that.)
Ultimately, only the Quarut is in charge of a crime that everyone pretty much agrees is a crime—as mentioned above. The Monster Manual inevitables go after pretty standard-fare crimes (though they tend to focus their efforts on astounding examples of them), and the varakhut protect divinity but that’s definitely not universally appreciated, not even by the divine when you consider how many god-killers are themselves gods.
Vestiges from the 3.5e Tome of Magic supplement are fairly difficult to concisely describe because they don’t really exist, per se—but they used to, and they want to again. The binder class operates by allowing vestiges to temporarily share in the binder’s existence, in exchange for supernatural powers. And the gods are often extremely uncomfortable about vestiges because vestiges’ non-existence puts them beyond the reach of gods.
The leaders of most organized religions are aware of binders to at least some degree. Most choose to keep that knowledge secret, lest the common clergy and worshipers learn of powers beyond the reach of their deities. Occasionally, a church even maintains a secret arm of its organization to seek out and eradicate binders. Such a force usually possesses a small library of texts describing vestiges and the practices required to summon them, so that its leaders can teach members to recognize the signs of pact magic and train them to defeat binders. Ironically, books stolen from such libraries introduce many future binders to pact magic. In fact, many binders began their careers as clerics before the promise of a swift means to power seduced them to the path of pact magic. This attrition is one reason why clerics, paladins, and other religious people who know about binders react to them in an unfriendly or hostile manner.
(Tome of Magic, pg. 14)
However, this only really applies to those few who know about binders in the first place, and even among that population only those particularly zealous about the supremacy of deities are particularly concerned by it. Prior to that description we have
Most people have an indifferent attitude toward binders because they know very little about what such individuals do. Even those who gain a basic understanding of binders’ powers typically view these individuals with the same respect or fear that they view conjurers or necromancers.
(Tome of Magic, pg. 14)
Feels related, even though it’s probably not actually relevant to your concerns:
Being made a Darklord of Ravenloft
The mists of Ravenloft occasionally snatch someone up and spirits them away to Ravenloft, where they find themselves the Darklord of a new Domain. Those who become Darklords are near-universally evil, and being a Darklord is an experience tailor-made to make the Darklord miserable. Ravenloft simultaneously pushes Darklords to act on their very worst impulses, and punishes those impulses brutally. If this seems like a tricky balancing act to get right, it is—Darklords are chosen very, very carefully to be the kind of person who might be susceptible to this kind of treatment.
But the mists are also inscrutable. We assume the Dark Powers control them, we assume the Dark Powers have some reason why they want Darklords to exist like this, but we don’t actually know. And since every Domain is tailor-made to the Darklord, it’s not really clear what crimes get their attention.
A few gods regularly spend time on the Material Plane because of what they are the god of—usually, in this case, wandering. Because they are literally a god of wandering and not putting down roots, their presence on the Material Plane is not nearly so damaging as it is for most gods.
Prestige classes were a 3e thing, classes with requirements you couldn’t meet at 1st level, and which rarely had more than 10 levels (3e went up to 20th level), so you had to multiclass to interact with them. D&D 4e replaced these with paragon paths (which everyone got at 11th level and which simply added to what your class did instead of replacing it), and 5e has turned a few of them (e.g. eldritch knight, arcane trickster, etc.) into subclasses for particular classes
Time hop and time regression are psionic powers in 3.5e; as far as I know, they do not exist in 5e. Time hop bumps someone a few rounds into the future, removing them from combat for the time being, and time regression undoes the last round. In other words, very small manipulations of time—and time regression is a 9th-level psionic power.
Vestiges are a concept from 3.5e’s Tome of Magic; see that book for more details. The vestige Ashardalon, Pyre of the Unborn is detailed in Dragon Magic for the same edition. Note that 5e’s Ravenloft material refers to the Dark Powers of Ravenloft as vestiges—this is basically incompatible with either the Dark Powers or the vestiges of prior editions. Regardless of what’s deemed canonical as far as that goes, Ashardalon definitely did not become a Dark Power.
Tharizdun is usually described as an “intermediate power.” However, it must be understood that this is the relative power of his influence on the multiverse from within the Demiplane of Imprisonment. Free, he would be much, much stronger—it took many greater deities to imprison him the first time.
Maruts—but not inevitables—predate 3e in D&D. The name comes from Hinduism, and earlier editions of D&D had maruts that were not especially similar to the inevitable maruts of Manual of the Planes. It seems that the inevitables grew out of a single line about the 2e maruts chasing those who tried to cheat death, but similarities pretty much stop there.
Inevitables also appear in D&D 5e, but their purpose has been completely ret-conned. I think this is pretty stupid and recommend ignoring those changes, but more importantly, it also makes them irrelevant to this discussion.
In 3.5e, “pact magic” referred to what the binder class did, making pacts with vestiges. It had nothing to do with the warlock class, despite the use of the word “pact” in some of that class’s description. 5e pact magic is very much a descendant of the 3.5e warlock, however, and again has nothing to do with vestiges.