The Forsaken Outpost is one of the hub maps of Hub 4: Castle of Grief. There are two items on this map that you'll need to advance in Hub 4: Gibbet: the Liber Oscura and the Daemon Codex. You will also find access to the secret area Hub 4: Desolate Garden, if you crossed the trigger line on Hub 1: Bright Crucible and pulled the switch in the secret area of Hub 2: Shadow Wood, Hub 2: Sacred Grove. Otherwise the portal will be blocked, unless you use the "casper" cheat code.