I mean some of the stuff you can find in the Ruin's arena.

litorgans

bossrightspawner

bosstentacle

(There is no 'leftspawner', as the 'rightspawner' can be flipped for placement on the other side.)

I griefed his arena with /admin.

You could use it for building, but it still has some of the boss code and may spray errors in your logs because no one was supposed to mine it.

Ruin's full sprite

(except for that small strip of white)