Family: Ma-Mutt is his pet, possibly a relation to Mumm-Rana
Powers: The full extent of Mumm-Ra's powers are unknown, but include: Shape changing, energy bolts, flight, and various spell books.
We don't really know that much about Mumm-Ra, other than he's been on Third Earth for a very long time. (Since it was called First Earth, whatever that means.) He's run the planet through power and kept order through fear.
The first time we encountered him was when he made my ship crash on Third Earth. Angry at being wakened he encased us all in molten lava. It must have been a magical lava, because we somehow survived an ordeal that should have killed us.
You know what's weird? When my grandmother was terrorizing Third Earth she didn't encounter him at all. She fought Mumm-Rana, but not him. If I hadn't seen the two side by side I'd swear they were the same person. Most of Mumm-Ra's disguises seem to be female, and no one really knows why this is. We've never asked.
What we do know about him is that his power lies in the pyramid and in his sarcophagus in particular. Every time he's beaten he goes in there and recharges. He's also immortal. Drop a mountain on him and he'll eventually recover. When Nightshade blew up his pyramid he went into a vegetative state, but even then managed to recover and helped us stop Nightshade's evil. Don't mistake his actions for kindness though. He's plotting, constantly. He wants Third Earth to himself again, without any Thundercats or other intruders, so you can bet he's not through with us yet.