Iduna was the sister of Nëya and one of the two elf caretakers that danced in the Blood-Oath Celebration and healed Eragon of his scar as well as altering his appearance. She and Nëya were tattooed with a dragon with rainbow scales that came to life through magic.
Iduna reappears alongside Nëya in Inheritance, where they once again dance so Eragon is able to contact their dragon tattoo, in order to change the Riders Pact.