History
Like every location found on the Beyonder's Battleworld following the events of the first Secret Wars, Leenn was the product of the subconscious mind of the Fantastic Four's Thing, [1] the Thing had decided to remain on Battleworld [2] after discovering he could change back and forth between human and Thing forms. [3] Each of these creations were based off some aspect of Ben's memories or personality, with entirely fabricated backgrounds and origins. [1]
The kingdom of Muab was a realm of white magic which was maintained at peace so long as their ruler presided over them, and never ventured outside of the kingdom for longer than a month, otherwise the kingdom would succumb to black magic. When their current ruler, the toddler known as Juneyar went out on a caravan and was attacked by minions of Grimm the Sorcerer, he was rescued by the Thing and his companion Tarianna. The couple got the child back to the kingdom of Muab before the deadline, foiling the Sorcerer's plans.[4]
Presumably, when the Thing later left Battleworld, Muab ceased to exist just prior to the planet self-destructing. [1]