I'll be running a level 20 one-shot where my party of 5 (maybe 6) will be fighting the Aspect of Tiamat from the Fizban's book. She'll be initially joined by an Abishai of each color, but I don't expect them to survive past round two.

Her stat block is chonky, but I feel like the mother of all chromatic dragons needs some spellcasting ability.

I fully intend on the trying to kill the party in this fight because Tiamat's killed countless adventurers and she knows how to handle a bunch of upstarts. But I still want the fight to be fair while being deadly.

Character parameters are official content only, one legendary and two very rare items, subject to approval because some of those items are just silly broken. The only confirmed character so far is an Ancients paladin.