This is how AOE Rain spells (Orange Spell Gem) have always worked.
It hits for 3 waves, but it has a maximum number of targets hit = 4 (you can, however, hit yourself all 3 waves even if the NPC limit of 4 hits is met)
This means; as stated by Kavious above:
1 Mob = 3 hits in a row
2 Mobs = 2 hits each (2x2=4Max)
3 Mobs = 1 hit on all 3, then 1 additional hit on the first mob in the stack (first mob is defined as the first mob hit on the first wave if they haven't moved and "rearranged" their order) (1x3+1=4Max)
4 mobs = 1 hit on all 4 (1x4=4Max)
Note: before Velious launch, pets used to eat these charges and report the spell immune text, so at least the patch were Verant fixed pets to not each charges from rain spells is fixed, before that these spells were almost entirely useless on P99.
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