Under extreme/idyllic conditions, could Cult of the Dragon necrofy Tiamat?

Question

I'm trying to assess the plausibility of whether the Cult of the Dragon could have grown strong enough to turn Tiamat into a Dracolich. Note I'm referring to the legacy cult under Sammaster, not the reboot that is bent on liberating Tiamat. As this excellent answer points out, I'm wanting to have a twist were Tiamat gets turned into a dracolich before, during or after the big fight, Tiamat would not willingly under go the ritual.

1. Tiamat is a god, dying isn't a major concern
2. As a god, it's subject somewhat to what its believers imagine it to be

Both are interesting and valid points, but I wonder if under cherry-picked conditions, Sammaster could have grown his cult strong enough to force Tiamat into the dracolich ritual. Perhaps this would be the culmination of the "doomsday via undead dragon prophecy" and would make a neat plot climax if successful. In his canonical inventory, there doesn't appear to be any artifacts strong enough to make this possible, unless I'm mistaken.

I suppose it's possible that the task is entirely out of the cult's reach, but I'd still like to examine what would be on order to give the cult its best chance at success. Maybe the world could end without an overt necrofication of Tiamat, but it would seem like it would steal some of the prophecy's thunder if there was a dragon entity that did not turn into a dracolich -- god or not.

Answer 1

No, Tiamat isn't a valid target for the rite

Tiamat is not a dragon, she's a god. Well, the 5e rules class her as a Fiend that takes the shape of a dragon, close enough, even though earlier editions had classed her differently. Her appearance doesn't make her a dragon. She has a stat block in Rise of Tiamat ¹ and doesn't have a creature type that is a valid target for the conversion into a Dracolich ²: The Dracolich template can only be applied to a very specific subset of dragons:

Only an ancient or adult true dragon can be transformed into a dracolich

Type. The dracolich's type changes from dragon to undead [...] ²

The Aspect of Tiamat ³, which is a manifestation of some of Tiamat's powers, at least is a Gargantuan Dragon (Chromatic Dragons), Lawful Evil, but it is most certainly not Tiamat herself. That's the most you can get for a dragon-Tiamat, but the Aspect of Tiamat is also not described or started as an ancient or adult Dragon. Usually, that age class would be in the name of the dragon, such as "ANCIENT BLACK DRAGON", but the Aspect of Tiamat lacks that statement. It is also more a manifestation of Tiamat's godly power than a true dragon. So under the rules for Dracolich, it doesn't qualify under the requirements either!

Since Tiamat is a Fiend and her Aspect is not an ancient or adult true dragon (and not herself either), she and her aspect are invalid targets for the rite. With an invalid or no target, it automatically fails. Tiamat will still be pissed and murder the cult.

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Note that the gods and deities in the 4th and 5th editions deliberately were very rarely given stat blocks, since players in earlier editions started to murder gods simply because they had stat blocks. One prominent school of GMs believes that not giving gods any stats, and treating them as unbeatable plot devices, is the better solution.

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1 - The Rise of Tiamat, p. 92-93.
2 - Monster Manual (5th Ed), p.83.
3 - Fizban's Treasury of Dragons, p. 166.

Answer 2

Trish’s fine answer already covers trying to just apply the dracolich ritual to Tiamat: that isn’t going to work. That ritual applies to mortal dragons; Tiamat is a god and so not a mortal dragon. But if we dig into the lore a little more (including older lore since 5e hasn’t gotten into a ton of detail on this sort of thing), we find that

D&D gods are what they are believed to be

If belief in Tiamat-as-dracolich¹ were strong enough, it could change her.

The problem here is that Tiamat is very, very old, and very, very well-known. The entire multiverse believes she is a dragon with five heads, one in each of the colors of the chromatic dragons. The Cult of the Dragon’s own beliefs about her can’t change that—their numbers pale against the beliefs of everyone who has ever heard of her, which is a substantial chunk of the population of the multiverse. Because they worship her, their beliefs may count for somewhat more than their raw numbers, but not remotely to that extent.

For this to happen, the Cult would have to convince a very large subset of the multiverse’s total mortal population that Tiamat is, in fact, a dracolich. This would include convincing a lot of beings that are very ill-inclined to listen to a mortal cult, most importantly actual dragons. Something like this is unprecedented for a god as well-established as Tiamat. More importantly, there’s no way it could just happen overnight—it would be a very gradual process, which Tiamat would inevitably catch wind of and then put a stop to.

Tiamat is a the god of chromatic dragons. Chromatic dragons have been around since very nearly the beginning of everything, and she is very, very firmly established as being what she is. That age, longevity, and establishment means that her divine nature has enormous inertia. Changing it would be cataclysmic—and given her age, we know it’s already weathered numerous cataclysms as it is, so we’re talking about not just any cataclysm here. Anyone capable of making this happen wouldn’t need to bother—they’d already be the most powerful being in the multiverse.

1. _{Or a dracolich-like deity, since being a god would make it still not quite the same thing—dracolich in the same way Vecna “is” a lich.}

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I maintain that the best, most lore-compatible approach to the scene you want is for Tiamat to consume the divine essence of another dragon god, probably Kalzareinad or Faluzure. Kalzareinad is my vote because 1. he is literally the god of dracoliches, and 2. he’s on extremely thin ice, for a god, and thus could plausibly be a target of opportunity for a desperate Tiamat.

Answer 3

It sounds like you're asking the wrong question: no, a goddess/fiend can't (or wouldn't) be turned into a lich per se, but you could achieve your "plot twist" goal by having her transform her physical aspect (either because her power "is strengthened in this plane" by a ritual, or because she gets really, really upset at something) to take on the appearance and powers of a dracolich, or other undead dragon, or really anything else that she/you could imagine.

In fact, https://forgottenrealms.fandom.com/wiki/Tiamat mentions an "Undying Queen" avatar that sounds like exactly this.