Bifrost is a massive rainbow bridge located in the southwest corner of the River Vid. Its north exit is a one-way and one-time trip to Asgard and the final endgame. Once the north exit has been used, the bridge collapses and may not be used again.
The only enemy type present is the edgewort, which will attempt to push the player off the bridge to their death. It is safe to melee them from above or below.
The friendly god Heimdall guards the north end of the bridge. When you approach him, make sure you have the horn Gjall in your inventory to complete his quest. Otherwise, his quest will fail, but the game may still be winnable if the player has completed several other quests.