Enslavement refers to the forceful and unwilling stealing of another nation's unit, as opposed to the somewhat-willing seduction of units that occurs in Charming. The biggest difference between Enslaving and Charming is that, if the Enslaved unit was a Commander (also known as a leader), they are reduced to troop status.
Enslavement is a Sorcery effect. The following spells enslave fellas:
, Thaumaturgy 9, affects the entire team, doesn't affect the
, Thaumaturgy 8, affects the entire team, only affects the
, Alteration 8, affects the entire team, only affects
Pretender Gods,
Prophets, and Disciples may not be Enslaved, of course. They can't be Charmed, either.