Help with enchantments that summon tamed entities

ok, so heres the deal. Im making an enchantment that can summon an entity to fight for you after you kill a mob/player. How do I make it so after the entity dies, the entity only has a CHANCE to spawn, and the entity will not attack me, just the ones i am currently attacking. I want this effect to also only happen while a sword has the effects of the enchantment i mentioned.