During the Battle of Killahead, Deya the Deliverer exiled and imprisoned Gunmar and his Gumm-Gumm Army in the Darklands, then used the Amulet of Daylight to seal the bridge so they could never escape. The bridge was then destroyed in pieces and its remains were scattered all over the world.
Centuries later, the son of Gunmar, Bular, and the Changelings began to gather the stones of the bridge to be able to reconstruct it and thus release Gunmar. However, the portal could only be opened by the Amulet and by the hand of the Trollhunter. They manage to force the current Trollhunter, Jim Lake, to open the portal. However, Draal manages to remove the Amulet from the bridge before Gunmar could be released, losing his arm in the process.
The remains of the bridge happened to be guarded in Trollmarket, until Jim decides to leave alone to the Darklands to destroy Gunmar. Having assembled the three Triumbric Stones, he uses the Amulet to rebuild the bridge and cross to the Darklands.