We have heroes who summon new allies and heroes who revive, why not combine them?
Bronze, rise from the grave: if a hero has died, Necromancer Summons two skeletons imitating the most recently died hero. They have low health and no skills but use the same weapon and same elemental damage type. Max of four skeletons.
Silver, deadly concoction: a powerful dangerous potion is thrown, gradually draining the opponents health and distributing it among all allies
Gold, unnerving presence: for the first ten seconds of battle the opponents have reduced attack damage and healing
Platinum, master of death: while deadly concoction is active the skeletons cannot die and gain additional armor