Keres

Keres (a.k.a. Daimones) are battlefield spirits of famine and disease that feed on violent death.The Keres were daughters of Nyx. In mythology, they were found wandering battlefields. In later times they were regarded as the vengeful spirits of the dead. They live in the Underworld and supported the Titans, but didn't directly attack Olympus.

The Demigod Files
In Percy Jackson and the Sword of Hades, Percy, Thalia and Nico are ambushed by a group of Keres. Percy descirbes them as part humanoid females, part bats; with pug-nosed and furry faces, fangs, bulging eyes, shriveled arms, claws, leathery wings, and stubby bowed legs. During the battle Percy is poisoned by one of them, and would have died if Bob (Iapetus) did not heal him. The way they poison is by scratching you and causing the poison to enter the bloodstream. On Percy, they scratched him on his exposed shoulder.

Powers

 * Keres feed on death which means their power increases when people die.
 * Have deadly poison that only a gods' or titans' powers can heal.