The Asgardian Collar is an Asgardian device used to vocally restrain prisoners.
Following the defeat of the Chitauri Invasion, the Avengers had managed to capture Loki, who had been beaten into submission by the Hulk. As Loki had continued mocking the Avengers, briefly taking the form of Captain America to mock him, Thor responded by locking his mouth shut with the Asgardian Collar and took him away, refusing to hand him over to Alexander Pierce along the way. The Avengers then took Loki to Central Park where Thor used the Tesseract to teleport them both back to Asgard where Loki would face justice.
Hunt for Lorelei
Lorelei was forced to wear this collar during her six-hundred year imprisonment. With the collar rendering her mute, Lorelei was unable to use her power of enthrallment to escape. When Lorelei escaped Asgard after the Dark Elf attack, she fled to Earth where she rid herself of the collar. Sif followed her, but the collar she brought was damaged during her skirmish with the Dogs of Hell. Leo Fitz was able to fix it. Sif eventually subdued her with the repaired collar, rendering her mute once more.