LogFAQs > #881314638

Sion's w passive is what makes him massively powerful end game, it eats the souls of his victims which increases his max health, which is why he synergises so well with Titanic Hydra (it does aoe damage based on your max health) and the soul walls explosion deals aoe damage based on his health too. It does however have to survive the initial 3 seconds of wind up without being popped before he can use it as an attack.

As for his overall passive, the resurrection thing, it basically turns him into a kind of zombie lich, his health drains at a static flat rate the second he gets up and he's given huge amounts of lifesteal and can activate an ability that gives him movement and attack speed for a few seconds. The cooldown on it is long enough for it not to be worth considering reusing though.
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I'm bad at Maffs.
BEDMAS Bioshock, Earthworm Jim, Diablo 2, Mass Effect 2, Ark: Survival Evolved, Super Meat Boy