Baldurs Gate 3 is oriented in 5th Edition. We can see in the game that a cambion transforms a human into a devil after he breaks a term of his pact. Is this possible in D&D 5th Edition? Normally you start as a lemure.
Similar things have happened, though something different is probably occurring in Baldur's Gate 3.
Generally speaking, when a devil makes a pact with a mortal in exchange for their soul, they're required to wait until the mortal dies in order to claim it, after which point it may be transformed into a lemure.
Examples of mortals becoming fiends
However, there is precedent for a mortal being transformed into a fiend while still alive. In Dragon Magazine #359, "Demonomicon of Iggwilv: Apocrypha", it is described that the demon lord Lascer was once a human who was transformed into a demon:
Using horrific methods best left undocumented, Obox-ob transformed Lascer into a minion, forcing a transformation from mortal to tanar'ri. He then sent his puppet out into the Abyss to serve, initially, as a spy—his humanoid form, Obox-ob hoped, would allow him to gather intelligence on the tanar'ri.
Another precedent is the marilith Kaliva, a human who was transformed into a demon by a powerful wizard with a shapechange spell, according to in the adventure module "Nemesis" in Dungeon Magazine #60. She was treated as a true demon in all ways and was able to earn promotion through normal Abyssal means until she became a marilith.
However, both of these are from an earlier editions of the D&D rules (very late 3.5, and AD&D 2e, respectively), and it's entirely possible for a 5e product or video game to alter or ignore earlier lore. This is also a demon, rather than a devil, so it's not related specifically to a pact.
A similar thing happened with the tieflings of Bael Turath, who were originally humans that made pacts with Asmodeus, becoming in effect part devil-blooded. However, tieflings are considered humanoids and not fiends, and this is in fact asserted by the relevant character in Baldur's Gate 3.
There's also the drow, who are often transformed into driders and other creatures by the demon Lolth. Driders, of course, are not fiends. As with the tieflings, what this establishes is the ability of powerful demons or devils to transform humanoids in general, though not explicitly into devils, or necessarily as punishment for violating a pact.
Baldur's Gate 3 in particular
What's unusual in Baldur's Gate 3 is (click to reveal spoiler):
According to Wyll's statistics, he remains human, and of humanoid type, after his transformation. He does not gain any new powers normally associated with being a devil, and doesn't become a tiefling, cambion, or anything else. It may in fact be that he's still human, but has merely had his physical appearance altered.
I believe Nobody the Hobgoblin's answer correctly deduces what it happening when this occurs in Baldur's Gate 3:
Baldur's Gate: Descent into Avernus describes that someone who breaks a deal with a devil may be punished by having their appearance changed to become a devil, and magic will detect them as if they were a devil, although there are no other game statistics changes. Any devil, even a cambion, can make a deal with a mortal, for which this is one possible punishment.
The only unusual thing here is that the cambion who holds a contract with Wyll is also his warlock patron, which is usually only possible for beings as powerful as archdevils, like Zariel herself.
It's possible for an Infernal Contract
For the sake of this question, I will assume that we are dealing with a cambion who had a devil as one of its parents, and that it would count as a devil1.
Devil Pact (i.e. Infernal Contract)
Pacts with devils are described in the Monster Manual on page 67:
a contract with even the lowliest devil is enforced by Asmodeus's will. Any mortal creature that breaks such a contract instantly forfeits its soul, which is spirited away to the Nine Hells.
And under Diabolical Deals in Baldur's Gate: Descent into Avernus (which states the contracts are enforeced by "the weight of the universe", not Asomodeus), consequences of a Breach of Contract are described on p. 114 as follows:
A character bound by an infernal contract who fails to pay the price specified in the contract immediately suffers a penalty for breaching the contract, as specified in the contract itself. Common penalties include the following:
- The character's soul is forfeit when the character dies (that is, the character is reborn in the Nine Hells as a lemure). [...]
- The character grows horns, a tail, or some other devilish features that can't be removed by any means short of divine intervention (though illusions or disguises can conceal them). As long as these marks persist, the character detects as a fiend when subjected to detect evil and good spells or similar magic. [...]
The first cited option would turn the soul into a lemure, and if the devil on the other side of the deal was of high enough standing (counting as an Archdevil or Greater Devil2), he could immediately promote the lemure into a lesser devil (MM, p. 67).
The second of these options here would transform the character into what looks like a devil, even though it would not change its creature type. I have not played BG3, but from the comments it sounds like this is what happens in the game -- changed looks, unchanged type.
If the Cambion were a Patron for a Warlock, there is no provision in the 5th edition rules how you actually can break the pact by not following its demands. Warlock is a Class, and the pact is what gives the character of that class its powers (PHB, p. 105):
A warlock is defined by a pact with an otherworldly being. Sometimes the relationship between warlock and patron is like that of a cleric and a deity, though the beings that serve as patrons for warlocks are not gods. [...] The magic bestowed on a warlock ranges from minor but lasting alterations to the warlock's being (such as the ability to see in darkness or to read any language) to access to powerful spells.
And from Xanathar's Guide to Everything (which are optional rules)
A pact can range from a loose agreement to a formal contract with lengthy, detai led clauses and lists of requirements. The terms of a pact- what a warlock must do to receive a patron's favor- are always dictated by the patron.
The pact is not described in a way that would hold a punishment for the PC if broken, but rather in receiving special powers in exchange of servitude and specific behaviour as long as the pact is valid. Since the Pact is what gives the Warlock its powers, if the Pact would be broken by non-adherence, the primary effect then would be that the Warlock character would lose those powers.
There are no provisions in the 5e warlock class on how to convert the warlock PC if they lost their warlock status. Therefore the game rules do not support breaking the pact, and turning into a devil as a result is not possible using 5e rules with a warlock pact.
1 Under core 5e rules, cambion is not strictly a devil to begin with. Its creature type is just fiend, any alignment, while all devils are of type fiend (devil), lawful evil. Mordenkainen's Tome of Foes on p. 21 adds the option of Infernal Cambion, and describes powerful variant Cambions that are the offspring of Archdevils, but even those only gain a different ability and by the rules do not change their tag.
Variant: Infernal Cambions
Some devils grant a unique ability to their spawn that replaces the cambion's Fiendish Charm trait; Zariel and Geryon have a penchant for spawning cambions to serve as war leaders among their followers
2 A mere Cambion as cited in the MM is just a CR 5 monster. It seems to be a bad match for an entity powerful enough to act as a warlock patron, which for the devil options of the Fiend patron is described as:
Fiends powerful enough to forge a pact include [...] archdevils such as Asmodeus, Dispater, Mephistopheles, and Belial; pit fiends [...] that are especially mighty
But maybe a special Cambion, child of an Archdevil, and acting as a war leader, might be powerful enough in standing to count.
I think your question is starting from an incorrect premise.
Technically speaking, in Baldur's Gate 3, the change is purely cosmetic. It doesn't alter the character's Type or give him any new qualities; he's still listed as Human on his character sheet. He's only been turned into a devil in the sense that he looks like one, not in any metaphysical sort of way. (And realistically, he just looks like a Tiefling, so I don't know what everyone's so worked up about...)
Can that happen? Sure. The DM can decide that an event in the game changes your character to look like a devil, turns your hair purple, or spontaneously develop a tattoo on your face. The DM could decide you turn into an actual devil, too, but that's not what happened in Baldur's Gate 3 anyway.
D&D 5th edition does not have specific rules for what happens when you break a pact, or how devils form. Devils grow in a particular way that is described, in general terms, but there aren’t game rules for it really. Just “an A eventually gets promoted and becomes B, and then later C,” and so on, without any rules for how, when, or why those promotions happen. A warlock player-character is not expected to ever become a devil, and if one did, they would probably become a non-player-character as a result. D&D 5e doesn’t really have rules for playing a “true” devil.
Older editions of D&D had more here (including rules for playing as a “true” devil, at least some of them), but even those didn’t really thoroughly get into these details. The terms of a pact and the punishment for breaking it are, in particular, very individual: the punishment can, effectively, be anything at all so long as the warlock agreed to it and the patron has the power to enforce it.
(Also, note that, to my understanding, Baldur’s Gate III plays fairly fast and loose with alignment—by which I mean it barely brings it up—and the distinction between demons, devils, and other fiends does not get a lot of focus. There are some important distinctions that, for example, mean that a cambion would basically never create a devil in the older lore. Some of this comes from 5e though, where “cambion” no longer has the more specific meaning it once did and instead is used to mean any half-fiend.)
Anyway, pertinently to your example, yes, it is possible to “skip” stages of devil-hood (and demon-hood, and presumably archon-hood or whatever though I’m not sure that’s ever been described). This is usually a reward for a warlock, as the lower stages are mostly pain and suffering and getting bossed around by evil incarnate, and getting to skip that means you get to be the boss torturer instead of the one bossed around and/or tortured. Devils really, really want to be the boss torturer, it’s pretty close to their raison d’être. Getting to actually become a devil with any kind of authority or autonomy would only be on offer for pretty powerful warlocks, where Hell wants to put them to work and needs them to be operative enough to do so. Though context could also matter; if the cambion needed a certain sort of devil at that moment, had a handy warlock with a broken pact that allowed such a thing as its punishment, that might have been that cambion’s choice. (I’m not familiar with the scene in question.)
The best example, perhaps, of how skipping stages is definitely possible may, ironically, be Orcus, a demon prince and famously a demon prince who climbed his way all the way to that station all the way from the very bottom as the soul larvae of a former warlock. Orcus is famous for this because the other demon princes did not do that; they skipped steps along the way (including, in some cases, skipping all the steps along the way, as Lolth did, and as most of the oldest Archdukes of Hell did). Other demon princes were former mortals, but they didn’t start out as larvae.
Also, in the deeper lore (per Faces of Evil for 2e, 1997), “starting out as a lemure” is... not really accurate. For one, most evil souls start out as soul larvae, and in most cases that’s going to be the punishment for breaking your pact, if not something worse. For another, the “natural” starting step for the exemplars of Hell is the nupperibo, not the lemure. This is something kept secret from everyone by the handful of highest-ranked devils that are “in on” the scheme. Everyone else, devils included, believe that nupperibo is a dead-end “punishment” promotion, where lemures can be made into nupperiboes which are slightly more powerful, but their only option to advance from there is to be rendered down into a lemure again. This is incorrect: nupperiboes form naturally in Baator, as the lowest form of “Baatorian,” as opposed to “Baatezu.” Baatorians are the native denizens of Hell, and mostly disappeared in ages past. Asmodeus and company took over a nearly-empty plane and became the Baatezu, and they need to do extra work to maintain their grip on it and prevent a possible Baatorian resurgence.