The Eternidad will allow its user, and anyone holding onto them, to enter Faerie undetected. The person must tell the artifact their destination, otherwise the artifact will choose one for them which may come with risks. It also protects the user from faerie enchantments, such as the lures of a revel.
The Eternidad is a piece of polished wood twisted into the infinity symbol. The artifact shines brightly, glowing with a dim, deep gold color. When used to travel to Faerie, its magic is quick and simple with no flourish; the person will simply disappear from their previous spot and appear in Faerie.
The Rosales family once had a close relationship with the Fair Folk, resulting in certain members owning objects charmed with or protected from faerie magic. The family called the heirloom the Eternidad, "to mean a time that has no beginning or end, like the time in Faerie." The artifact is said to be a token from Rhiannon, and is given freely as a gift by a family of hadas whom a member of the Rosales family aided.
During the advent of the Cold Peace, the Rosales families were told to surrender these gifts to the Clave. The Cohort was after this particular heirloom. To prevent the heirloom from falling into the hands of the Dearborns, they falsely claimed that only a Rosales could make the object work, and Jaime Rosales took the heirloom and went into hiding.
While at the London Institute, Dru Blackthorn touched the heirloom among Jaime's things and was briefly transported to the weapons room in the Unseelie Lands before Jaime left again with the heirloom. Shortly after, Cristina contacted Jaime, wanting to borrow the artifact so she can go to Faerie with Mark and Kieran. The artifact successfully transported them to Faerie and back, and Cristina became the new custodian of the heirloom.