Aurelion’s Strengthen Minion

Seep your mana into one of the undead minions under your command, and bolster their resolve. 

Choose a target undead minion you control that is either a skeleton or a zombie, and, through the power of necrotic energy, transform them into a stronger undead. 

If the target is a skeleton, it becomes a Minotaur skeleton. 
If the target is a zombie, it becomes a Ghast. 
After 1 hour, the creature’s body disintegrates into a pile of ash.