He-Ro came He had great strength as well as great magic ability, as we all know. He pretty much acted like He-Man meaning he saved Eternia from various evil threats, and generally protected the Royal family...the Grayskulls.
Eventually He-Ro married into the Grayskulls.
Many years later one of He-Ro's descendants. King Grayskull (yeah the one from that really cool Power of Grayskull Episode) has inherited his ancestors immense physical strength. Then one day the Horde attacks and he also learns that he has some magical ability, and using this newfound power he sends the Horde to Despondos.
As he dies he seals his soul and his power within his sword until the time the chosen one is found.
Even more years later we get Adam, prince of Eternia and descendant of Grayskull. He is giving King Grayskulls Sword of Power by the Sorceress and told that it is his destiny to use and become He-Man to once again protect Eternia from the threat of Evil.