i always thought of dark grace or whatever it's called as being his big ability, as last i checked he was the only champ in the game that got to do 9 unpreventable and global damage on deploy
Grace of the Dead, Mindwipe Aura, Dark Favor, and Death Touch were all unique to Xulos when he came out, I think. Four new and unique abilities on a champ is unthinkable now, and while each of those have been passed out in one form or another, Xulos still reigns with DF3. Also, he's much better now.
even if he still had all those abilities (as various upgrade path choices) they wouldn't be unique, so I'm not sure overly unique would be the issue.