The delay is said to be as a result of player feedback participating in the closed beta for the content.
We received tons of great feedback during our beta of Dying Light – Hellraid. Big thanks to everyone who participated! We’re hell-bent on applying your suggestions, so we will be launching the new game mode on August 13 for PC, and August 14 for consoles. pic.twitter.com/3rqp680b7P— Dying Light (@DyingLightGame) July 17, 2020
Hellraid features four player co-op and requires Dying Light to work. Dying Light itself is also four player co-op.