If in the Demon Hunter starter quest you choose to sacrifice yourself and not the other demon hunter you... anti-climatically walk back to your corpse as normal. However when you do it triggers a message from Illidan commenting how interesting it is "you also have an immortal demon soul".
A) What does this say about all those Paladin player characters and what have you? Probably nothing, but damn could this all go somewhere.
B) They actually came up with an in story reason why Illidan keeps getting brought back and it's actually a little clever.
C) When did Illidan figure out he could Res? He calls it having a "demon's soul" implying it's post-Skull (whether he's right is again up for grabs). He says this before his Boss battle. Did Arthas actually kill him?! Or did he find out even sooner. Perhaps this isn't a recent power, he could have whisped back on battlefields as far back as the of the War of Ancients without even himself realizing.
Video- Illidan explains "Get back in your corpse!":