Evil beholders can take control of enemy unit, but only to make them attack other unit (after that control ends but enemy unit cannot move in that turn). You can take control in such way only if enemy stack overall leadership is less or equal leadership number that is shown at ability description (more evil beholders you have=more max leadership number is). If evil beholder attacks there is possibility to put enemy in sleep (that part LiveInABox described correctly
).
Chance to put enemy in sleep:
lvl 1 - 100%
lvl 2 - 50%
lvl 3 - 25%
lvl 4 - 10%
lvl 5 - cannot put to sleep