Portal wither skeleton farm

Voltrox's build of ianxofour's instant wither skeleton farm. You find a fortress inside a soul sand valley, lay a nether brick platform on the chunk centre, and the wither skeletons that spawn on it walk into a nether portal. They come out on an overworld bridge, queue up there, and a second portal drops them into a killing chamber on the nether roof where you stand. Nothing but the platform can spawn them, so the mob cap belongs to the farm alone. There is no redstone in it at all, and the skulls, bones, coal and stone swords land at your feet.

Wait 5 to 10 minutes after you switch it on. The overworld bridge has to fill with skeletons first, which is what nearly every "nothing spawns" comment turns out to be.
On a server it only works if you are the only player connected, or at least the only one in the overworld. Anyone else more than 128 blocks from the platform makes them despawn on the way.
If they reach the portal but never drop into the killing chamber, build the nether roof platform one block lower. Klicks, caseyr803 and several others all landed on the same fix.
Do not light the portal at 4:46. When you light the one at 7:47 and step through, you arrive in a cave packed with the skeletons that already came across. memis0 died in one and a half seconds in netherite and lost a hardcore world.
The spawning platform has to be nether brick, not any other block, or the pack spawning does not happen.
Turn render distance to 2 and simulation distance to 5. graythingie got a lot more spawns out of that alone.