As of now, all co-op features of Grim Dawn are unconfirmed with the exception that the game will have co-op throughout the main quest as well as co-op challenges.
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Grim Dawn is an action role-playing game intended for digital distribution on the PC. It is being developed by veterans of Iron Lore entertainment, the makers of Titan Quest. Grim Dawn is being built with Crate's own improved version of the Iron Lore engine and toolset; the same technology used to create Titan Quest. As a spiritual successor, Grim Dawn will attempt to better perfect the magical formula of exploration, character advancement, and loot collection pioneered by Diablo.
Players will be thrust into the dark, war-torn world of Cairn where a once proud empire has been brought to ruin and the human race driven to the edge of extinction. Cairn has become ground zero of an eternal war between two otherworldly powers, one seeking to use human bodies as a resource, the other intent upon destroying the human race before that can happen. This cataclysmic war has not only decimated human civilization but is warping the very fabric of reality and, in its wake, giving life to new horrors.
For humankind it is the dawn of a grim new age where iron has replaced gold as currency and the importance of salt as a weapon makes it far too valuable to waste on food. Small enclaves of human survivors exist scattered throughout the world, holed up in hidden refuges. These humans have quietly watched the warring invaders destroy one another and have become wise to the strengths and vulnerabilities of their otherworldly foes. A few survivors have begun to exhibit strange new abilities after surviving possession or exposure to the warp. These unnatural powers are feared by some but give many new hope of launching a resistance to fight the "outsiders" and reclaim what's left of their world.