Set sometime before the formation of New ThunDera:
It was a normal day on Third Earth and Lion-O had been out and about for most of it on patrol and was getting quite tired by the time he decided to head back to the lair to hear what the others had been doing. He hadn't had much luck finding anything on either the Mutants or Lunatacs and that had been a source of frustration. Mumm-Ra had also been quiet too so that was another source of concern but the lion never did find anything.
*******
Unbeknownst to the lion, Mumm-Ra had been watching him all day and had a plan that would rival any of his others so far. All he needed was to catch Lion-O and as it happened, that wasn't too hard to do considering how tired Lion-O was and that he wasn't watching where he was going when he collapsed into a pit. Mumm-Ra flew over as a small insect and sprayed it with some sort of substance which put the lion to sleep within moments. The mummy waited a bit to make sure the effects held and snatched up the lion and took him back to his pyramid and sealed him inside it but not after removing a lock of the lion's mane.
The mummy used this to craft a spell and soon had a form of the lion before him but the only big difference was the eyes which burned red and had none of the warmth Lion-O was known for amongst his friends. Mumm-Ra smiled evilly.
"Go out and seek those that were your friends and find ways to end their lives and those of the Mutants and Lunatacs as well and then come here to reap your rewards," he said as the lion growled and left the pyramid at a run as the mummy cackled. He had given the lion some powers of his own so he could blend in and not be seen.
******
The first warning anyone had of the killings was a day or so later when Red-Eye was found dead with his body burned in a ditch close to where Skytomb had been seen. Luna had demanded answers but there were none to give as their scanners hadn't found anything or picked up anyone.
The second warning was when Jackalman was found garroted to death in the swamp near Castle PlunDarr and that had the Mutants equally freaked out as well. They were trying to find out themselves and that got the attention of the Thundercats who had noticed that Lion-O had been missing for a good while and had gone out to look for him.
Mumm-Ra had been smart enough to put a fail-safe into the false Lion-O so he appeared when the Thundercats went looking for him out near the Berbil village. Of course, the two murders had reached the Thundercats and Lion-O was worried too but at the same time he was calculating something and none of them would catch it till it was too late.
*******
The next days were torture for just about everyone as people kept dying. Chilla was drowned in a sea of rock salt, Alluro was pitched off a cliff and Luna had her neck broken. Tug Mug had been ripped in half but by what or how was unknown. Amok had gotten away and no one had any ideas of where he had gotten to. Slythe, Vultureman and Monkian all met about the same fate as Jackalman except Slythe's tail had been cut off.
It was also about this time that Cheetara began to suspect that something was wrong with Lion-O who kept having periods of blackouts and not knowing why. What made it worse was that the blackouts were happening at about the same time as most of the killings so far and that wasn't boding very well for the Thundercats who had a meeting while their leader was supposedly sedated in the med-bay. The only ones not present were Lynx-O, Pumyra and Cheetara who were trying to find help from elsewhere and it may have been a good thing as the fake Lion-O woke and massacred everyone in the lair except the kittens who escaped and warned the other three.
"That's impossible!" Cheetara screamed when she heard but deep in her heart, she knew. Snarf and Snarfer had been away at the time and had missed it.
"Wait a minute," Lynx-O spoke up. "What if this was something else that happened?" he asked. "We know that Lion-O went missing a few weeks ago before the killings happened and then there are the blackouts he had. Is it possible that there's two of them out there?"
"Where's the real one?" Pumyra asked.
"His eyes were glowing an evil red when he came into the council room and he kept muttering something that we didn't understand," Wilykit said, her eyes full of tears. That gave the other three the clue they needed.
"Mumm-Ra has our lion with him I'll wager," Cheetara grated. She, Pumyra and Lynx-O left the kittens with the Warrior Maidens for the time being as Willa and the Berbils were handling the clean up at the lair. Strangely or perhaps not, Tygra and Bengali weren't in that bad of shape but Panthro was a lost cause as he'd been garroted and his spine snapped.
******
Back at the pyramid, the fake Lion-O had come to claim his reward and Mumm-Ra presented him with the real Lion-O, imprisoned in the cells. The Sword was in a chest in the main room of the pyramid and Cheetara and the others could sense its urging call the closer they got to the pyramid.
"That's our answer," Cheetara said as the three slipped inside. They found two Lion-Os battling it out inside, one with the glowing red eyes and the other was their leader though he was at a disadvantage.
"He needs the Sword!" Lynx-O yelled. "Find it and get it to him but be careful around that other one!" The three split up and the fake Lion-O tried to take them on but couldn't, seeing as they were working together to keep him back.
Lion-O was able to get around to join Cheetara and he kept the fake one from attacking her by using the claw shield which he still had with him. She found the chest and broke the lock on it just as Mumm-Ra appeared. Lion-O jumped between the mummy and cheetah and took a knife to the gut as he grabbed the Sword and sent up the call. It sent Lynx-O and Pumyra into action and they were able to kill the fake and then attack Mumm-Ra before he was defeated. Lion-O groaned as Cheetara stayed with him and the puma came to help.
"I had no idea what Mumm-Ra was doing but I heard most of it," Lion-O gasped weakly.
"Rest for now," Cheetara said, her voice quavering. Jaga appeared and helped heal Lion-O and everyone saw him that time but said that Panthro was gone along with the Mutants and Lunatacs.
"He won't be arrested for this will he?" Lynx-O asked.
"Show Mandora what transpired here and he won't be," Jaga told them. "I can't guarantee what will happen but know that there is evil in all of us, but it's up to us if we let it take control."
"Agreed," Lynx-O said soberly. Jaga left and the group spoke to Mandora who let Lion-O go but with a warning not to leave Third Earth for a while until the ruckus died down which was understood.
At the lair, there was healing to do on the parts of Bengali and Tygra and explanations given which took a few days and Cheetara was a source of comfort for Lion-O who was upset but glad that his friends had forgiven him and were looking past the problems. Tygra was subdued but grateful to know that the fake Lion-O along with Mumm-Ra had been destroyed and for a while there was peace for everyone.
The End
