Eerie Jack

The Chanter's Sunken Rune is a piece of coral from underneath the ocean waves.

Source
The item can be purchased in East End Police Station (heroes) or The Gotham Tap Room (villains) during The Witching Hour event.

Deep Sea Chanter
Upon activating the trinket, Eerie Jack is summoned to aid the player in battle. When Eerie Jack dies nearby enemies take damage.

Attacks:

 * Fire Ball - does a single attack for 40 damage (on training dummy).
 * Fire Beam - does four consecutive attacks, each for 14 damage (on training dummy).