How to increase the chance of a spell's success against a foe that can save vs spells most of the time

Question

I'm trying to work out a way to debuff an opponent in order to increase the chance of a spell succeeding against it. First of all, let me describe the situation:

The opponent in question is a Rakshasa-turned-Lich, with (AFAIK) the resistances and powers of both, so it’s not an easy one to defeat. What's more, this fiend-lich has created multiple phylacteries (in a horcrux-y sort of way) so fully destroying this foe is probably not a realistic option and the best I can hope for is an indefinite imprisonment somehow. For context, the PC is a high-level Fiend/Blade Warlock who should have access to a 9th level Mystic Arcanum by the time the confrontation takes place. Already-taken Arcanums are Soul Cage, Plane Shift and Demiplane. (The choice of what that 9th level Arcanum will be is currently unresolved — possible choices include True Polymorph or Gate).

Current ideas revolve around something like one of the following:

1. True Polymorph (creature-to-object) followed by sealing that object up in a demiplane and throwing away the key;
2. Gate spell to create a portal to Carceri, push the enemy through, close the Gate.

Option 1 would seem to be the more permanent of the two, but of course True Polymorph grants a saving throw, which the enemy would in all likelihood succeed in (+9 to WIS saves, with advantage, and that's before we consider the Legendary Resistances of a Lich, whereby it can simply choose to succeed at a saving throw 3x per day).

Option 2 doesn't involve any saving throw, as such, so that's good — but any magical means of pushing the creature through the portal tend to be low(ish) level spells or modification of low level spells. For example: Bigby's Hand, or Repelling Blast added onto Eldritch Blast. (Physically forcing it through the portal might work, but comes with a serious risk of ending up on the wrong side of the portal along with the enemy, so it's not ideal).

We should assume that this will be a 1-on-1 conflict taking place on the Prime Material Plane (though it could also potentially take place on a Demiplane created by the spell of that name).

That's the context. My question, then, is this:

I've been scouring spell lists (PHB, XGtE, TCoE, MToF) for possible high-level debuffs (since any spells of less than 7th level are a non-starter) that might reduce this enemy’s near-impenetrable magic resistance and allow a high-level spell such as (for example) True Polymorph to have a better chance of taking effect, but I've failed to find anything.
Can somebody point to something that I might have missed, either RAW or from pre-existing lore?
(Asking as a player)

Answer 1

Don't try to out magic a Lich

Liches literally died for magical power. And when you add on the Rakshasa features, you are trying to put out a bonfire by pouring water from a watering can.

So don't use the water, use the can!

Beat them into submission

Instead of magic, use muscle to beat the creature unconscious¹. Any magic attack you do will be lethal, but you can declare any melee weapon attack as non-lethal and so when the creature reaches 0 hit points, they do not die but merely stay unconscious. Since this is a lich, you do not want it to die as it will just reform somewhere else.

Now that they are unconscious, assuming they follow the same rules as everyone else, you have one to four hours to imprison them until they regain 1 hit point and become a threat again.

Undead prison

Stop spell casting

Your goal at this point is to prevent the creature from magically leaving the prison and/or dying so they can reform outside of the prison. The best way to do this is restraining the hands and mouth. For the verbal portion, create a form-fitting, silver muzzle to prevent speaking and restrict any verbal component spells². Next, fasten the arms/hands/fingers to a silver yoke so they cannot wiggle their fingers to do any somatic component spells. They should be stripped of everything to stop any material component spells. Use silver as it's cheap and easy to forge to the correct shape quickly. It also won't break down like some other metals.

The lich has a large number of spells at its disposal, but the ones we need to be concerned about are damaging spells (so it can't kill itself) and teleportation spells like dimension door and plane shift. But they are still limited to the rules of spell casting and need to follow the rules of how components work. The biggest fear is dimension door as it is a verbal-only spell, so that means without sight, sound, or knowledge of where they are, they can just go 500 ft in a random direction to escape anything we try. The Rakshasa has innate spell casting with removes the need for material components, but they still have to be concerned with verbal and somatic. For them, it would be plane shift that becomes a threat.That's why the prevention of verbal and somatic extremely important.

Encase them

Now that you've restrained them to prevent spell casting, you need to restrain them completely. The simplest solution would be to just encase them in cement.

Due to the lich's_"Undead Nature"_ feature, it "doesn’t require air, food, drink, or sleep." So they won't suffocate or starve by being stuck in cement. I would recommend a 30' x 30' x 30' cube just for reasons to be explained.

With another spell caster's help you could also do transmute rock and swap between stone and mud. Fill a 30' x 30' x 30' room with stones. Place the lich on top and transmute the stones into mud. Since the lich is unconscious in our scenario, they would auto-fail the Dexterity check and sink into the mud. Once they have sunk deep enough, dispel the mud back into solid stone.

Put them away and throw away the key

The final step is to put the cube somewhere where they should not be able to escape from and no one can get to them. A simple solution would be the demiplane spell; just create a room for the cube and never open that same demiplane again once the lich is secure. By placing them in a demiplane, it acts as a safeguard against dimension door. They cannot travel 500 feet if there is nothing/nowhere to teleport to³.

How it all comes together

1. Create the muzzle and arm/hand/finger yoke from silver so they are ready and close to the right size.
2. Create a demiplane that will be the final resting place. You will need to cast this multiple times over the course of this solution as the prep takes time.
3. Line the 30-foot walls with with 1" lead plates to block most magic spell (specifically divination), leaving a hole for the door. Keep a spare sheet to cover the hole/door later.
4. Placing the door on the "ceiling" of the room (so you're looking down into a pit), fill half the room with cement.
   • Or completely full of stones if using transmute stone.
5. Fight. Hopefully win by knocking them unconscious.
6. Attach the muzzle and yoke; adjust as needed to be form fitting.
7. Once again, open the demiplane with the door in the ceiling.
8. Lower the lich in (no fall damage), placing them in the center of the cement "floor", exit, and fill the rest of the room with cement.
   • Or transmute the stones to mud so the lich will automatically sink in, then dispel the spell to change it all back to stone.
9. Place the final sheet of lead where the door is.
10. End the demiplane spell and hope everything works.

The goal is, they cannot move, cast spells, hurt themselves. If they can get free enough to cast spells, nothing should allow them to teleport out of your box. You've filled up the entire space so there is no gap outside the block but still inside the demiplane.

Further options

1. Add glyphs of warding to the silver yoke to trigger some sort of warning if they are tampered with. But you would need another spell caster for this.
2. Pin a Periapt of Wound Closure to them so if they do manage to hurt themselves, they automatically stabilize instead of dying.
3. Use Dimensional Shackles to further restrict teleportation.

---

¹ There's been talk of "Can the undead even go unconscious?" As written, there is no rule saying they can't; it's not listed under "Undead Nature" ability and it's not listed in the Condition Immunities. So while it may be a popular house rule, my answer is based on what the rules allow. Admittedly, the whole Rakshasa-Lich hybrid is well into homebrew territory so it's possible that the OP has also decided that unconsciousness isn't an option. We can't be certain.

² Being undead means they don't need to breathe, which is a big factor when alive creatures talk. So there is no way to know if just holding the jaw shut of an undead would stop it from using verbal components. If it does, great. If it doesn't, there are other safeguards added to the prison to help.

³ Per this question, demiplanes exist inside of the Ethereal plane. Since dimension door cannot cross planes, it could only be used to move within the small demiplane.

Answer 2

An Elaborate Mundane Prison

The best 9th level warlock spells to imprison a foe that can't be killed by normal means are true polymorph and imprisonment which both require saving throws.

The typical way to increase your odds on these is to wear out Legendary Resistance on other debuff spells and use bestow curse to cause disadvantage on the key save. These don't work on a rakshasas.

Instead consider simply imprisoning the creature in an elaborate mundane prison after knocking it unconscious using a melee attack:

Sometimes an attacker wants to incapacitate a foe, rather than deal a killing blow. When an attacker reduces a creature to 0 hit points with a melee attack, the attacker can knock the creature out. The attacker can make this choice the instant the damage is dealt. The creature falls unconscious and is stable.

It shouldn't be too difficult at high level to devise a prison that can't be physically breached, and you can bind the wizard so they can't use somatic components of spells (limiting their spellcasting considerably).

The real trouble is the potential of escape through the rakshasas innate plane shift. For this, we will need an anti-magic field, the most permanent of which is found on the beholder:

The beholder's central eye creates an area of antimagic, as in the antimagic field spell, in a 150-foot cone.

If you restrain the beholder in such a way that the antimagic area engulfs the imprisoned rakshasas, you can prevent any spellcasting options for escape.

Answer 3

Silently wrestle them down and tie them up

Pushing them into another dimension does nothing, they have access to plane shift. Carceri may make ot more difficult if your DM uses the optional rule that is mitigates escape via spells, but there are surely other ways to leave that plane that a creature this powerful should be able to find. It is a prison, but it's not inescapable at all:

Secret ways out of the plane exist, but they are hidden and well guarded by traps and deadly monsters.

Spells, as you have identified do not work well. We have a similar foe. Even for my wizard, who is a Divination school one with Portent, legendary resistance is just too hard to chew through, and here it is even worse because they are immune to anything up to 6th level -- you are a warlock with just 4 Arcana of better than 5th level, so you can expect to get of a single spell - if they do not counterspell it, and then they still get a normal save.

The overall plan here, similar to the other answers, is to knock them out and imprision them instead. The big problem with this plan however is that they can plane shift away when things get dicey. Why should they stick around waiting until you knocked them out? As a lich, they are probably super smart, and like many a villain, may try to escape when they realize they are about to lose the fight. So stopping that is the first order of business.

Step 1: Don't let them escape

Lich-Raksasha has innate plane shift from his demon side, needing no material components, on top of access to normal plane shift and dimension door as a lich, the latter of which just needs a verbal component. (I would not be surprised if they had a custom spell selection like teleport on the lich side either, given they are homebrew anyways).

You cannot surprise them when they sleep, and quickly gag and tie them, as liches never sleep.

What you can do is get some help with setup. Silence can make it so that they cannot cast most of their spells, in particular the ones like plane shift and dimension door have verbal components. Use your demiplane Arcanum, and find a high-level cleric of who can upcast silence to level 7¹ into a Glyph of Warding in it with a trigger of when the Lich-Raksaha is witin range. When you encounter them, get your demiplane, and open the door, to shut off most spellcasting.

Step 2: Grapple them and knock them out

Learn to grapple well (neither Raksasha nor Lich is proficient in Acrobatics or Athletics; you can get Athletics with a feat pick like Skill expert, even with Expertise; if your Strengtht is low, uncommon gauntlets of ogre power can help, too.). Grapple them and keep them in the silence zone.

You then can knock them unconscious by dealing non-lethal damage (p. 198 PHB), without fear that they just plane shift away once they are bloodied. Or, if your DM house-rules undead cannot be knocked KO, you can try and slap manacles and a gag on them while you have them in headlock. That will probably require some more grapple checks, but should be feasible.

Step 3: Handicap and imprison them

We have a similar foe. Our plan is to imprison him in a dead magic zone instead of killing him. There are several in the Forgotten Realms, for example one in an old wizard tower in Waterdeep.

If you do not have access to such a zone, cut off its hands and part of its tongue, so it cannot cast somatic or verbal components, but keep it alive (or undead, for that matter).

A Lich Raksasha still needs components and the Raksasha only can get out of the requirement for material components. That means also for them, if you can block their ability to speak and gesticulate, their spellcasting will not really work. (And if you can blind them, many spells requiring sight won't. I am not sure if you can blind a lich -- they have no real eyes -- but a leather bag or locked up metal mask without eyeslits should work fine). Handicapping them like that before locking them in should make it impossible for them to cast any spells.

¹ Overcoming Rasasha Spell Immunity

The Raksasha Limited Magic Immunity says

The rakshasa is immune to spells of 6th level or lower unless it wishes to be affected.

And the spellcasting rules say under Casting A Spell at a Higher Level:

When a spellcaster casts a spell using a slot that is o f a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level.

So you can overcome that protection by casting your spells using your 7th, 8th, and 9th level slots. You yourself of course cannot, as a warlock, but allied spell casters that prep you can.