Realm of the Dark is a survival scenario for 1p/2p which comes in three modes (cooperative/solitaire/competitive) and four levels of difficulty (Normal/Challenging/Difficult/Nightmare). Cooperative mode is 2 players teaming up to resist and conquer the enemy waves. Competitive is a PvP mode which lets the players compete against each other as in who can survive the longest without being conquered by the enemy waves. Solitaire mode is the one player mode of the survival scenario. Players can pick either slot two or slot three in solitaire mode.
For feedback, you are requested to head to the forum topic provided or leave the feedback in the #modding channel of the official BfW discord server with preferably pinging the add-on author, knyghtmare#8753.