Initial hit on the main target is 100 potency. Then the 200 potency is split between any targets around it still alive, including the main target.
Thus, if 1 target only (the main one) then that target will take 100 + 200 potency damage. If 2 targets, main target will take 100 + 100 potency and 2nd target will take 100 potency. If 3, then 1st target is 100 + ~67, 2nd target is ~67 and 3rd target is ~67. And so on. It's overall damage won't really increase regardless of how many targets there are, and will do more damage per target the less targets there are.
