The Hungering Destroyer is a single phase "patchwerk" style encounter in which the raid must form small groups to help their allies heal up, as many people gain Gluttonous Miasma debuffs which causes them to become healing immune aside from leeching health from their allies. The boss has many mechanics which heal him, however these can be countered via good tank coordination and raid positioning!

The Hungering Destroyer is the second boss in the Catacombs wing, and is unlocked after defeating Huntsman Altimor. Defeating The Hungering Devourer unlocks Lady Inerva Darkvein. 

Hungering Destroyer Quick Tips in Castle Nathria

• Tanks taunt off of each other after each Overwhelm, as this also helps keep Growing Hunger stacks low.

• Players with Gluttonous Miasma are immune to all incoming healing from player spells. They can still be shielded.

• At least 2-3 players should help soak a player who is afflicted with Gluttonous Miasma. This should be increased if the player needs a lot of healing in a short time.

• Healers should keep the raid topped when Desolate and Consume is cast.

• Players marked by Volatile Ejection lines should spread to avoid hitting any player with more than one line, ideally hitting no other players with their line.

• Spread out away from others when the raid is afflicted with Expunge.

• Move the boss to the edge of the room and run away from him when he casts Consume.

A full guide can be found on Wowhead: https://www.wowhead.com/guides/hungering-destroyer-castle-nathria-raid-strategy-guide