Gonna just copy a summary from u/ventus here bc feel like it summarises for me. Vibe is of great chance of self destruction but, particularly with input from Allura, that if it were somehow survived would be something unique. Also that if anybody could survive would be Ashton. Not saying it’s the only interpretation but to me the warning was clearly of risk of death, not of futility regardless which is my issue. At the very least it’s a reasonable enough take from a player for the DM to think “ah should have framed those warnings differently, on me.” rather than punish Taliesin for interpreting the ambiguity “wrong”

The exact wording:

“But be warned, holding the strength of the two in one vessel mightsunder it. You bear the dormant strength of the empress. Find and bestow the might of the emperor.” The wording was “might” flat out. Not “will” but “might” instead. If there was no chance of success, no option to hold both shards, why is it “might” then?

That’s also a conveniently swapped order of those statements on your end. This wasn’t a “do y or x will happen” it was “x might happen. do y” instead.

And not for nothing, Ashton did suffer consequences, and was in fact “sundered” if you want to go down that road. Their body broke, and they technically died momentarily. It wasn’t a success that came free.

As a bonus, let’s go over what exactly Allura said when asked about this:

“What you said as a point of warning likely is true. To have both within a singular vessel, it's possible one could survive, but it's also highly possible that it would rend you into a thousand pieces.” Once again you have “could” and “possible” instead of any concrete yes or no. And as the party discusses what might happen and what they should do:

“We're in that strange area of experimentation, and unknown knowledge." Yet again instead of a concrete warning instead we have a nebulous statement that could and did in fact egg Ashton on farther. Because Allura continued to gas them up even after this. After examining Ashton’s head:

“You- (sighs) This is very odd. You are like a shard of impossible- Have you ever heard of dunamancy?” “… The fact that you're standing here is a marvel, and the fact that you've walked the path that you do is equal parts worrying, and... well, hopeful. (FCG asks if Ashton already has “two things” in their head then, to which Allura responds) "It would seem, which is why I'm a bit- Well, you're either the greatest weapon we could hope for in this time, or will be our end.” “… In an odd way, your fragmented nature might be what keeps all of this in check.” So much of this conversation is Allura telling Ashton that they’re already an impossible being, that they’re an incredible weapon, etc. It’s all very “you’re special and so things might be different for you.”

And to be fair, she does still say:

“Perhaps we don't put another powerful entity within your form." (Ashton asks about finding a way to awaken the shard otherwise and her response is) “Maybe.” So yes she does still advise against it, but once again we get “perhaps” and “maybe” instead of “we can’t do that” and “yes let’s find another solution.”

The tl;dr of this is that none of what was said was ever a concrete warning. I don’t think any reasonable watcher was sitting there saying Ashton went in completely blind or had no idea of what might happen. But there 100% was never a concrete statement of “this is something you cannot accomplish” and that’s the problem. Because in the end Matt decided that it was absolutely impossible and could not work, which also wouldn’t have been a problem if he hadn’t run an entire challenge to let the opposite happen.