She probably couldn’t, and certainly wouldn’t
In addition to questionable appeal to a being that is proudly draconic and already immortal, godhood is going to cause some problems with becoming a dracolich.
On the first, dragons don’t have a lot of respect for dracoliches; they are seen as not true dragons. For dragons, that’s about as damning a criticism as you can get. Moreover, this attitude probably comes from Tiamat herself—she probably feels this way more strongly than any other dragon. And since being a god is a far better way to be immortal than being a lich, it doesn’t seem very tempting.
There are also technical issues here. As a god, Tiamat is a creature of belief more than she is flesh and blood—on some level, she already isn’t a true dragon, though of course she is something that certainly counts in the eyes of all dragons (even those that hate her would count her, because Bahamut’s in the same boat). She probably can’t become a dracolich per se—at best, she would do something visually similar while continuing to be a god. But this is a problem, because it makes us ask why she would do so—dracoliches get some things out of it. Does she?
But the really, really big problem here is that D&D gods are, and must be, what they are believed to be. So if everyone—particularly her worshippers—believe her to be a dragon—not a dracolich—that is what she is. Gods cannot change that easily or safely; most cannot even fathom trying. Of those who have tried, most have been destroyed. Those that survived, lost enormous amounts of power in the process. The most infamous case probably being Jergal,¹ from the Forgotten Realms. He was the Lord of the End of Everything, quite possibly the most powerful god of death in the multiverse (and Death is a big portfolio). He schemed to pass that mantle on—a scheme being necessary because a god can’t “just” retire—and even though he handled it expertly, it cost him nearly all of his divine power. He had been so powerful that his mantle alone turned three mortals into two greater deities and an intermediate. Afterwards, he was a demigod—that is an almost incalculable loss.
And Tiamat would be aware of that. Becoming something other than what she is believed to be—and being a dracolich, or anything like one, is very much not what she is believed to be—would be a monumental loss of power. And unlike Jergal, who was tired of his position, Tiamat loves being Tiamat. She would never do it.
What to do, then?
We already established that Tiamat isn’t going to be using the same ritual as a regular dracolich here, because as a god it wouldn’t work on her.
So what you want to do here is lean into that—and make whatever she does be something worth this side effect.
What would that be? Absorbing more divine power. Gods can take each other’s divine power; some gods specialize in it. That divine power comes with portfolios and beliefs attached, though—if Tiamat absorbs another god’s power, she is no longer just Tiamat: she is also, to an extent, the former god she’s absorbed, and what that god was believed to be.
Which solves all our problems in one fell swoop. Tiamat gains power, and also becomes something different from what she was, adds new things to what she was believed to be. This is a way around all the problems with godhood. And it’s very, very obvious why she would want to do it.
Gods I thought of:
So have Tiamat absorb the divinity of one of the undead gods. A few options spring to mind for me:
Vecna is popular these days, though after the Sigil fiasco he might not have enough power to interest Tiamat. On the other hand, that might be a good thing: taking too much power is much more dangerous, even assuming you can get it, just because it’s going to have a larger influence on who you are and are believed to be. It also might work well for the fight, since presumably you don’t want Tiamat to go from a challenging fight to a complete curb-stomp.
Orcus/Tenebrous would have been perfect, but that divinity was destroyed with great prejudice long, long ago. Actually, on second thought, while Tiamat would have loved to have it, there was no way she was going to get it.
Myrkul seems like a sweet spot though: he was one of those who inherited Jergal’s power, but he was destroyed by, and lost it to, Cyric (who lost it to Kelemvor). Now he’s reborn, but less as god of death and more as god of the fear of dying—still a big deal, but not nearly so much as the real thing. His apparent destruction and re-birth works to Tiamat’s advantage, because it means relatively few believe in Myrkul anymore. Gods depend on their worshippers for sustenance, so that makes Myrkul vulnerable. That’s a reasonable target for Tiamat.
Better options from a friend
I spoke with @afroakuma, who is a far greater expert on the subject of D&D lore than I, and he pointed out a very, very important reason why Tiamat wouldn’t become anything like a dracolich:
There is already a god of dracoliches.
Or, maybe several? Or maybe they’re actually aspects of the same god? Or maybe one died and passed the mantle on to one of those others who turned out to actually be an aspect of yet another?
Once upon a time, at least, there were two evil draconic deities much closer to dracoliches than Tiamat: Faluzure and Kalzareinad. But, at least if you’re in the Forgotten Realms (and The Rise of Tiamat is), Kalzareinad might have died, Faluzure might have actually been Null, Null might have picked up the dracolich portfolio when/if Kalzareinad died. Or maybe it was Kereska that got that portfolio. (For more on why there is so much confusion here, see ².)
Point is, someone is the god of dracoliches. And it ain’t Tiamat.
For the purposes of this plot, I vote (and afroakuma votes) for Kalzareinad. Unlike Faluzure/Null, who are big, big gods (or is one big, big god), Kalzareinad is small. Tiny. Barely holding on. An even easier target than Vecna, actually by a considerable margin.
And in possession of precisely the portfolio we want here. Kalzareinad is, or at least was, the dracolich god of evil draconic magic. It had been said that he alone knew the secret for creating dracoliches. But dracoliches are few and far between, and neither dragons nor the undead are great worshippers. And outside those few, Kalzareinad is deeply obscure. Almost unknown. Almost without believers.
That makes him an easy snack for Tiamat. Something she maybe could just flip a switch on for a quick power boost. Low risk. But it might have some visual effects, as Tiamat suddenly becomes the goddess of dracoliches and evil draconic magic, in addition to being the goddess of evil dragons.
Actually acquiring the chosen divinity
Tiamat can’t just randomly yoink another god’s divinity.
Once you pick one—of these, or another undead god, there’s quite a few—you have to figure out how Tiamat got her claws on their divinity. Tiamat would have to scheme a lot to be able to have this in her back-pocket during a fight—even the greatest of gods need to be careful going after even the weakest of other gods. Also, best case scenario is still not a clear win for Tiamat—even though she increases in power, she becomes something different from what she is, which isn’t often something gods see as a positive thing. So I think a plausible plot here is that Tiamat has been scheming to undo another god, and maybe leverage their divinity for something. Maybe use it personally, if she can work out how to do so safely; maybe barter it; maybe use it for some spell or ritual. Lots of options, many probably better than just slapping it onto her own spark and seeing what happens.
But then she’s losing a fight. An important one. One that has a bunch of other schemes riding on it. Maybe it’s time to just throw caution to the wind and flip the switch. Gods don’t often do that—and what you fight in The Rise of Tiamat is almost-certainly an avatar and not Tiamat herself, personally, for real, so it’s not as though the stakes are necessarily life-or-death here. (And as the case of Myrkul shows, death doesn’t always stick for gods in the first place, particularly not one with as established a following as Tiamat.) But maybe Tiamat is at a point where she’s pretty sure she can handle it, and maybe her schemes really need to not go off the rails at this particular point. So maybe she does. And it becomes an epic fight for your players.
Details on Jergal and his “retirement” scheme, and granting his power to “The Dead Three,” as Bane, Bhaal, and Myrkul were known, can be found in Faiths & Avatars.
The canon gets very muddled here, because in 1990, TSR published Draconomicon (note, the 1990, 2e book by that title, not one of the other times D&D has used it). That book decided to ret-con a whole slew of draconic gods, mash them together and split them apart and other silly things—in the Forgotten Realms only. So we have a strange divergence of canon, where everything else kept the draconic gods that had been around for a while, but FR had the same gods with different names and maybe some of them were actually two names for the same god, and so on. And then TSR died and Wizards of the Coast bought D&D and has never bothered to go into such detail again. But Null still shows up occasionally. I mean, so does Faluzure, so hard to say what to make of that. Kalzareinad doesn’t, but then he was always deeply obscure.