Raven Flock

Raven Flock is a monster that the Overlord can summon as his servant using the Call of the Ravens card. It cannot be picked in an open group during setup, but counts as a minion monster of its own monster group when it is summoned.

Note that strangely, the Raven Flock is not considered to be flying, and is thus subject to environmental effects such as Lava, Water, or Obstacles.

Description
Weak at first, the Raven Flock can be upgraded by the Overlord to be stronger and to have various abilities. All upgrades to the Raven Flock are lost if it is destroyed, but the Overlord can summon the servant again, never giving the heroes a moment's pause.