After experimenting extensively with the Horrors, I’m pretty sure they’re designed to be used with the Grandmaster trait “Death Nova”. Focus down one person in a large group of players, let them get insta-gibbed, and it’s an extra stack of AoE poison to add to your conditions. This doesn’t quite make sense, however, as having a 5-point minor trait only do something when taking one specific Grandmaster major trait- and not a particularly powerful combo, either- I have to conclude that this skill was most likely overlooked in testing. Most of the Necromancer traits are a tad underwhelming in comparison to others, but this skill is useless in all but the most specific of situations.