A Life Mask is a Caal techno-magickal device which disguises its user's magickal aura.
During the Ascension War, the Caal Scyr instructed its host in the creation of a life mask in order to infiltrate Avalon without being detected by Vin Dane, who was using the Orb to watch the system for Caal possession. Scyr's life mask was only a crude imitation of the genuine article, and was made by sacrificing an awakened Mage in order to "steal" his Aura and bind it to an amulet. While this proved sufficient to bypass Vin Dane's safeguards, the Orb itself was able to see through the deception when confronted by Scyr in the Umbra.
A true life mask, if any remnant of the original Caal technology still exists, may be considerably more sophisticated and versatile.