UNDOCUMENTED MECHANICS
It removes 25% of the target's current life, not its maximum. Cast repeatedly on the same monster and each hit takes less than the last.
The 25% does not scale with skill level. Points only widen the radius; the percentage is fixed.
The damage is typed as lightning, so the target's lightning resistance reduces it, and lightning-immune monsters take nothing.
There is a life floor: once a monster is at or below that share of its maximum life the skill stops working on it (Normal 0%, Nightmare 33%, Hell 50%). The floor is per difficulty and bites hardest in Hell.