Void Infestor

Bestow {1}{b}{b}
{b}, {t}: Void Infestor loses this ability and becomes an Aura enchantment with enchant creature. Attach it to target creature.
Enchanted creature gets +3/+3 and has "At the beginning of your upkeep, this creature fights another target creature you control." It becomes a creature again if it's not attached to a creature.