The Asgardian Collar is an Asgardian device used to vocally restrain prisoners.
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.