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 or for the former, and a for the latter.
Magic beings are specifically susceptible to a number of astral spells:
Additionally, some items such as the have bonuses against magic beings.