Ok, with Bow's new bio I think I have the new canon figured out....
Before Adam can fully transform into He-Man, he needs to be re-united with Adora and they both must tap into the power of Grayskull at once. At this point, he uses the techno-vest and unpowered sword of He.
The Sorceress, Duncan and Randor both believe Adora to be dead, killed by Hordak to Prevent the prophecy from coming true and Adam becoming He-Man to oppose his invasion and rule over Eternia...
*The past- Hordak to Keldor before he steals the Twins*:
*Hordak; through big hologram sent from Etheria*:"The son of Randor must not become a He-Man...
*Keldor; before he steals the twins*: Yes, my master...
But unknown to the Sorceress, Duncan and Randor, Adora is alive, raised by Hordak and serving the Horde. Eventually she discovers her true past, the treachery and evil of Hordak; and escapes to join the rebellion. She is eventually caught and imprisoned though. But right before she is caught she sends out a desperate message to Eternia...
*Hologram message appearing to Duncan*
Adora's Message: "Duncan, you bravely served my father, King Randor, during the Great Unrest; and now I have desperate need of your help. I have been captured and imprisioned by the Horde Empire. To free the people of Etheria and Protect Eternia, the Horde Empire must be stopped...
Help me Man-At-Arms, you are my only hope..."
Duncan shows the sorceress the message. Unsure of whether or not it is a trap, they do not inform Randor to spare him the devastation if it is a trick. But this is too improtant to ignore, as the uniting of the twins will unlock the power of Grayskull. So they are able to create a device that can send two people to Etheria and back. Naturally, they will send Adam, but The Trollan overlords demand that Orko accompany him so he can fulfill his original duties.
So Adam and Orko are sent to Etheria, with Adam being told, and under the asumption they are there to rescue an ally that needs help. He is not told he is there to rescue Adora, as he was not even aware he even had a sister, or that she was "dead".
So when they arrive on Etheria they come across a rogue and braggart: Bow. Adam knows they will need help on this quest and tries to get his help, as he hears this Bow is the best fighter around. So he tries to get Bow to help him infiltrate the Horde prison to rescue Adora. At first Bow refuses as he is not in the business of suicide missions. Then Adam persuades him with his royal riches...
*Adam*- "You know, where i come from, I'm a prince, do this for me and the reward will be great!"
*Bow*- "How Great?"
*Adam*-"Well, a lot greater than you can imagine!"
*Bow*- "I don't know, I can imagine an awful lot..."
So they all head to the Horde Prison, destroying some Horde Troopers and stealing the armor to disguise themselves. They infiltrate the prison with Orko as their "Prisoner", and put the place on lockdown. While Bow and Orko hold off the Horde Troopers trying to get in, Adam rushes to find Adora. Eventually he finds her cell...
*Adora*- "Aren't you a little short for a Horde Trooper?"
*Adam*- "Wha-? Oh right, the uniform...*takes off helmet*... I'm Adam Randor and I'm here to rescue you!"
The group eventually escapes the prison and find themselves surrounded by thousands of Horde Troopers and Hordak riding Mantisaur.
*Hordak sees Adam*- "You won't escape me this time!"
While Bow is busy fending off the Troopers, Adam and Adora face Hordak. Orko throws Adora the Sword of Protection secretly given to him by the sorceress and tells her and Adam to hold their swords aloft and recite: "By the Powers of Grayskull... we.... have...the.... POWER!!!!"
They do and fulfill the prophecy by channeling the power of Grayskull. They both destroy the swath of Horde Troopers to Hordak, and using the swords in tune, shoot energy at the Mantisaur, causing it to explode and sending Hordak flying away.
Bow "Great shot kid, that was one in a million!"
They retreat to the rebellion's secret base and they are all given medals, and Adora reveals who she is to Adam. And Adam is glad he didn't try to make a move on his sister.
Adam and Orko return to Eternia, with Adora needing to stay to free Etheria. But now with the power of Grayskull unlocked, the Horde Empire can be driven back by She-Ra. Bow joins the rebellion too, and they wave Adam and Orko goodbye.
Adam returns to Eternia and tries to tell everyone the story of what happened, but he needs to hide his identity as He-Man so it is forbidden. He even has to hide from his parents that his sister, their daughter is alive, in order to keep his secret. For if it is discovered he is He-Man.... uh.......
I don't know.
"Duncan never told you what happened to your uncle, did he Adam?"
"He told me enough... he told me you killed him!"
"No... I am your uncle"
"NOOOOOO.... wait, really?"