Ultimates:
Ellywick Tumblestrum
Ellywick's Song
"Hear the tale of dungeons... and dragons!"
Grand Master of Flowers
Bahamut's True Form
(not gained by use of ability, but by accumulating seven loyalty counters to turn on his static ability)
"I suppose it's time for me to get serious."
Lolth, Spider Queen
Lolth's Eight-legged Strike
"I eight all my opponents alive."
Mordenkainen
Mordenkainen's Library
"All the manuals you could ever need."
Zariel, Archduke of Avernus
Zariel's
Relentless Assault"Attack! For victory and/or death!"
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Others:
The Deck of Many Things
Down on the Deck
"Lucky draw!"
Vorpal Sword
Snicker-Snack!
"He left it dead, and with its head / He went galumphing back."