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At the time it was worded as a triggered effect ( when this creature deals damage, its controller gains that much life), but it has since been reworked into a static effect (damage dealt by this creature also causes its controller to gain that much life)

10+ damage out of nowhere kills people

You don't need a bunch of ramp

Since it turns back into a land when the turn ends, it's tricky to kill with spells, so it'll often stick around being a nuisance till the end of the game

Heliod triggers off each instance of lifegain, so Soul Sister cards like Soul's Attendant and Soul Warden can distribute tons of counters in a single turn cycle