This one-off special featured an extra-long adventure starring both He-Man and She-Ra called "The Third Sword". Horde Prime finds out about the existence of an ancient sword with equal power to He-Man and She-Ra's- except this is evil power, as it is called the Sword of Evil. The leader of the Elders cut it in half, and hid one half on Eternia, the other on Etheria. Horde Prime summons Hordak and Skeletor and sends Skeletor to Etheria and Hordak to Eternia.
On Etheria, Light Hope summons She-Ra and tells her of the Sword of Evil, and reveals one half locked in a room in the Crystal Castle. He tells her to take it to the Cosmic Well and throw it in. The Cosmic Well is atop an undersea mountain, so Frosta freezes the water and She-Ra blasts an undersea tunnel and slides down to reach the well. But she is attacked by Skeletor, who escapes with the sword and delivers it to Horde Prime.
She-Ra instantly contacts He-Man, and The Sorceress reveals the other half of the sword in a locked room in Grayskull. Ancient books say the sword should be thrown into the Pit of Light should the forces of evil ever look for it, but the Pit of Light is within Snake Mountain! He-Man makes his way to the evil lair, but is attacked by Hordak, who takes the sword and delivers it to Horde Prime. He-Man and She-Ra quickly journey to Horde Prime's base, and after a struggle, destroy the sword before Horde Prime can join its two halves.
(It was not often that we saw He-Man and She-Ra together in the comics, so this is essential for any collection. Also has a very good storyline, with lots of action and a great scheme by Horde Prime.)