The damage caused by Plague happens on the unit's turn, and is in the same amount as from a Poison spell from the same caster. Despite the similarities between this and Poison, this is not a 'mass poison' spell: creatures can suffer from both Poison and Plague at the same time. When the spell is cast, all creatures are targeted, friend and foe alike. Fortunately for the caster, Necropolis units are immune (even the living ones). Also, , , and constructs are unaffected.