This skill seems to be bugged. Although all enemies take the 70 damage, one enemy will take much more than this. The targeted Margonite seems to gain the amount of health the severely damaged enemy takes.
Because of its long casting time and the short duration of Abaddon's Chosen, the Margonites will very rarely use this skill to good effect. Even when they are able to use it, it is so easily interrupted that they rarely manage to cast it. If, however, the Margonite manages to activate this skill after casting Abaddon's Chosen, the maximum duration of it is 4 seconds.