The Spiritforge is the place where the magic of the wizards all began, as well as the stronghold of wizards. It's also the location of their last stand.
After the summoning the Forgotten and the ensuing outbreak of demons, many fellow wizards abandoned their wizard towers and fled into the Spiritforge. The Forgotten collected all six replicas of Gem of Eternity, which she corrupted. After placing it near the shields of the Spiritforge, she killed the Gembearer, hid the true Gem of Eternity, and lured his fellow wizard to collect a corrupted replica. When the wizard from the main story activated the Gem of Eternity replica, it siphoned power from the other corrupted replicas and destroyed Spiritforge's shield, allowing the Forgotten to finally invade the stronghold.