Call of the Ravens
When the Overlord exhausts the Call of the Ravens card, he places the Raven Flock token in a space adjacent to a monster. The unfortunate monster suffers 4 in the process.
The Raven Flock can be upgraded by the other cards in the Unkindness class, but these cards have to be drawn from the Overlord deck normally, and goes into the discard pile after the Raven Flock has been destroyed.
If the Raven Flock token is already on the map when Call of the Raven is invoked, it is moved from it's current location to the new place where Overlord has summoned it.
It doesn't matter how much Health the targeted sacrificial monster possesses when the Overlord uses Call of the Ravens, so it is often better to pick a weak monster that is already hurt.
Because Call of the Raven is used "at the start of your turn", it will often be put into play at the same time that the Overlord gains reinforcements. By being clever with what order the Overlord takes these events, the Raven Flock can often be gotten "for free" by having the monster that dies for it's summoning be reinforced right afterwards, provided that there was no other monster that could be reinforced.