The Rejuvenation Rack is an unlockable Rare item in Risk of Rain 2.
Completing the Naturopath Challenge unlocks this item.
All healing is increased by 100%, effectively doubling the healing received. Each subsequent pickup increases this amount by 100%.
Visually, the Rack is placed on the characters head, and looks like a pair of glowing green antlers. The Engineer, however, wears this on the grenade launchers/booster instead.