Heart of the Deep - KyaniteMods/DeeperAndDarker GitHub Wiki
The 
heart of the deep is an item used to light the Otherside portal.
The heart of the deep randomly beats when in the player's inventory.
Obtaining
Hearts of the deep can be obtained by killing the warden, with a 100% chance of being dropped.
Usage
Lighting up the Otherside portal
By using the heart of the deep inside a reinforced deepslate frame, an Otherside portal will be created. This is similar to using flint and steel in an obsidian frame, which creates a Nether portal.