“Alternative League” is a mod that replaces the Dungeon faction.

Exactly. Seers have terribly low single-target damage but high damage against formations and dense groups, practically without retaliation. They are complete garbage when attacking unit by unit, but Flame of Vengeance is a counter ability to locking. As stated in the description, Seers deal their damage to a target. Again, this damage is applied to everyone if they are moving. And again, double damage is dealt to everyone around the target, except for the target itself, which constitutes their real damage. As strange as it sounds, the target for the attack should not be the enemy's priority units.