Eldor is the name of a planned character for the cancelled The Powers of Grayskull toyline.
In the planned storyline, Eldor was (the 1st) He-Ro's mentor and the guardian of the Book of Living Spells (which was supposed to come as an accessory for the figure).
MVCreations MOTU Comic Series
Eldor has a cameo in issue #5 of volume 3 of the MVCreations comic book as part of the Council of the Wise.
Masters of the Universe Classics Bios
Eldor finally received an action figure in the Masters of the Universe Classics toyline. He received a bio and was featured in several additional character bios and in minicomics.
R’tendril the Wise was a great but cantankerous sage and powerful wizard of Preternia. After discovering He-Ro,an amnesiac young Cosmic Warrior, lying in a smoldering impact crater, R’tendril healed him in a mystic pool that unintentionally caused the technovirus to spread throughout the planet. A grateful Ro became R’tendril’s pupil and took to calling him Eldor. During their adventures, Eldor and Ro helped the time traveler Dare, owner of the Sword of Power in the future, when he came to Preternia. Together they taught him magic and combat skills so that he could become a true hero. Although Eldor had little patience with the boy at first, he later expressed pride in Dare’s progress before he returned to his own time. Along with He-Ro and the archer Sharella, as well as King Grayskull, Eldor confronted many villainous forces in the turbulent world of Preternia. When He-Ro perished during the First Ultimate Battleground, a grief-stricken Eldor used his knowledge of transformation to submerge the Three Towers into Subternia. He prophesied their reemergence when great kings returned home and the past and future intertwined.
2012 DC MOTU Comics
Eldor has a cameo appearance in the 2012 DC MOTU series mentoring He-Ro, here depicted as King Grayskull's son who gets terrible revenge on the Gar, who were presumed to have betrayed Grayskull due to an insidious plot concocted by King Hssss.