[EternalSpearman]

In the vein of what mlepinski said, to cast a ritual (one not cast in a battle) spell:

Click the word "Defend" (or whatever the current orders for the commander is) under the picture of a mage, and click "Cast Ritual Spell", and choose one from the list. Some are summoning spells that summon a particular creature or creatures, some are global enchantments that effect the world as a whole.