Server Ideas

Hello there. I have a pretty big question I’m trying to solve, but so far, no idea.
What I’m trying to achieve is a 256-man raid in a mmorpg I’m developing. It’s already around alpha, with most of the content, but the multiplayer part is just destroying all my brain cells. So far, i was able to achieve multiplayer on open-world using channel based, but I literally have no idea how to achieve 256-man raid on a single map. I was thinking of breaking the map into chunks and render each one on a different part of the server, more like channels, but sometime, there might be parts where they will “collide”. I avoided any battle with more then 32 players in it, to avoid spell lagging, but still, for example if they are running into the last encounter, they will have to get everyone into a room and each part of the raid to go into a separated door.
My problem is, how in the world would I host 256 players on a single map?
I know it sounds pretty impossible to set 256-man on a single map, because of lagging, frame dropping and stuff, but is there any workaround of it, so I could somehow put all 256 players on a single map and somehow divide them, while they would “still be there”?