Wither cage wither rose farm

A wither rose farm for the End that does not need the end gateway broken, built by cnuz around cubicmeter's wither cage. The wither is trapped in a small cage and targets an iron golem, its skulls fly out and kill the endermen collecting on the platform, and each one dies to the wither effect and leaves a wither rose. Bubble columns separate the skulls by the angle they enter at, so the side heads do the killing and the middle head's rare blue skull goes somewhere harmless. It is the design people reach for on Paper servers and anywhere the mushroom trick for breaking the gateway does not work, and it leaves your gateways intact.

Replace the lightning rod where the wither is summoned with an iron chain. mathmchd's wither escaped with the rod in place and he confirms the swap works on 26.1 and up.
iggo11k adds an obsidian protector on the pillar, built on 26.1.
Do not log out near the wither and do not teleport back next to it. crybaby8723 warns that is how it gets loose, on Paper servers in particular.
The bubble columns sort the skulls by their entry angle. ryanl5565 works out that only the middle head produces blue skulls, which is why the side heads are the ones doing the killing.
Blue skulls destroy obsidian as well, so rebuilding the cage in obsidian is not a fix by itself.
The tutorial cuts between steps a lot and several builders lost the block counts because of it. Thornios and chair5613 both raise it and cnuz agrees in a reply.