This spell allows a mage to call forth a creature, so long as they have killed many of its ilk. Take caution when summoning creepers. This is a spell only to be used by the darkest, most cunning of necromancers.
While creating a summon spell, you will be asked to place a Crystal Phylactery into the altar. What is a crystal phylactery, you ask?
A crystal phylactery is, basically, a magic bottle.
A phylactery is filled by killing many of the mob you desire to summon. The first mob killed while an unattuned Phylactery is on one's person is the mob to which the phylactery will attune itself. It is not compatible with Bosses, save for one: the Golem from the battle towers mod. Be warned, however, that the Golem does not take kindly to being summoned.
Once a summon spell has been created, it can be cast using any shape. Casting the spell will summon the mob the phylactery contained the souls of. If you attack, or are attacked by, a creature while your summoned minion is with you, it will attack said creature if it is able to. Peaceful beings, such as pigs, cows, or sheep, will not attack these creatures, as they have no method of doing so. They can be interacted with as normal (cows and mooshrooms can be milked, for example), but items that would normally drop upon their death will not. Mobs such as Horses or Pigs will, however, drop equipped Saddles or Horse Armor.
Bosses can not be captured in a phylactery, and as such, can not be used as one's bodyguard, so to speak. The phylactery will work on all non-boss mobs, including those from other mods. This would allow one to, for instance, summon Knight Phantoms from the Twilight Forest as allies. Keep in mind that summoned mobs AI is identical to that of a tamed wolf. This means that summoned skeletons would not fire a bow, but would instead run at and attack their targets. Summoned creatures will not attack creepers, for obvious reasons. Please note that if a creature is usually hostile toward the player, it will not take kindly to being attacked.