This item can take many forms, though almost all of them are no bigger than a human hand, such as necklace pendants, enchanted gems, or even intricately-carved wooden figurines. Invariably, each has runes on them in some esoteric language--for example, Celestial or Primordial--which provide a nondescript blessing.
The item must be attuned by a creature to be used. No prerequisite is required for attunement.
When the attuned creature sleeps within 10' of the item: for the period of its normal long rest, it does not physically age. The creature can, at the end of the normal long rest, consciously choose to continue to slumber for some time--while it does so, its physical age actively reverses at the same rate (growing younger by one hour per hour, for example). While the totem is being used this way, it glows faintly with a color unqiue to each totem.
Nobody alive actually knows what happens if one attempts to physically reverse one's age to before their birth. The implication that nobody is alive to say so has been widely interpreted as "something bad happens".
Only gods of the Life domain can consistently make these totems (by bestowing their blessing upon a mundane object of appropriate size, a pact sealed by the runes inscribed on its surface); their clerics sometimes succeed in requesting one.
Finding such totems is the lifelong goal of some vain creatures who wish to extend their lives unnaturally; evil wizards (especially those prone to becoming liches) and evil dragons (especially red dragons and followers of Ashardalon) are notoriously common seekers. Some scholars have been known to speculate on the "disappearance" of such dragons, only to have them come back some centuries later looking far more rejuvenated than natural.
The object retains its usual AC and HP. When destroyed, it simply becomes ineffective.
Per Detect Magic, the totem has a strong aura of both enchantment and necromancy.