Fels

A Fels is a dungeon mob found in Catacombs Floor 5 and above, which rarely drops a Fel Pearl. It spawns in floor 5 or higher, appearing as an enderman head on the ground. When someone gets within close proximity of the head, it will turn into a Fels and start attacking.

Strategy
The Fels is very painful, so one strategy is using some sort of range to clear out the area around it without provoking it. This will allow for the player to 1v1 it, making the fight considerably easier

Another method is pretty much the opposite, leaving mobs around it and using a croud control weapon such as the Flower of Truth or Spirit Sceptre to life steal from them.

Endgame players, however, will be able to kill these effortlessly due to their high damage and ehp.

Trivia

 * When you get close to a Fels, their nametag changes to "Dinnerbone".
 * Although it's an Enderman, it can be hit by arrows and fishing rods, however it will still teleport erratically.
 * It is recommended to have the beserker or mage handle this mob, as if the archer does not one-tap it, it will begin to teleport making it hard to hit.