@Jaredan: There's no Champion that's 100% unkillable, right?
It seemed to me like Xerath was pretty impossible to kill, but I'm gonna guess that Riot will just go "nope, he can die, but only to very specific means".
Though I will probably argue that Aurelion Sol is unkillable, or at least won't STAY dead for long even if you do destroy him. Why? He's a Star Dragon, that's why! [slayer-jinx-wink]
EDIT: Actually, only exception to this rule would be Kindred, because how do you kill death as a CONCEPT? But to counterbalance it, in League's story, they'd have no physical form.
is 100% Unkillable, but he will die eventually... due age.

technically doesnt "die" being the concept itself but it does and an end like everything
Soraka's description says she was a celestial being, but gave up the immortality due to WW.
Bard is a celestial being, and most likely has that immortality.
Given that he is a magical construct..
They are from an immortal race. Given the war they can still die.. but they also possess immortality.
He isn't stated to be immortal though.
Iceborn are immortal.
He would make sense to be able to live through anything.