The Bifrost Bridge
The Bifrost, or Rainbow Bridge, is a gateway or portal connecting Asgard to other Realms. Most commonly used to travel to the other Eight Worlds, it can be tethered to other lands through the use of rituals.
The Bifrost was, historically, guarded by the Aesir god Heimdall. Since the raising of Asgard, Heimdall is still missing, and the guardianship of the bridge has fallen to Magnus.