Maybe the more health you have, the more things you can summon?
If you are at low health, you can summon one or two zombies/skeletons, high health, five or six.
Maybe you can "unlock" powers- the ultimate power summons a giant bone creature to protect you (look at the CTF Battlegrounds save).
Other unlockable powers could be summoning two skeletons with scythes, skeleton knights and soldiers (weapons do extra damage), or summoning Zombie Tanks.