Magic Being
A magic being requires magic leadership. They will rout if all such commanders leave the battlefield.
Units that are both magic beings and undead instead require undead leadership. However, it is the reverse for units that are both magic beings and demon, instead requiring magic leadership.
Good examples of this would be a 
Manikin or Mandragora
for the former, and a Ghul
for the latter.
Magic beings are specifically susceptible to a number of astral spells:
- Arcane Bolt
- Control
- Arcane Domination
- Unraveling
- Opposition
- Celestial Chastisement
Additionally, some items such as the Moon Blade have bonuses against magic beings.