What is the best way to idiomatically translate this pun into English?

Solution 1:

Two ideas came to my mind.

One is you could change the word chair to house, since house (esp. "big house") has been used as slang for jail or cell for quite some time.

This big house is for the Dark Lord, so he can house everyone.

The other idea is more grim; sometimes, The Chair is slang for "the electric chair."

And this chair is for the Dark Lord, so he can sit everyone in The Chair.

I realize these aren't perfect: one switches words from chair to house, the other implies capital punishment instead of prison. However, translations of puns is a tricky endeavor; that was the best I could think of. Maybe someone else can do better.