All nine totems are lit, and the inner sanctum is underwater. Part 5 opens with a plan to move the bed down closer to it. The party, going by the nameplates and chat, is Basket, JJBonsig, WonderSim and dash10138. Basket has night vision and water breathing going, and the area is supposed to be lit up and "safe-ish."
Safe-ish
It isn't. Within three minutes Basket is blown up by a Drowned Creeper. The bed was never actually put down, so the respawn is back up at the temple entrance. The bed goes down on the next trip, "so next time I die I'll be closer." Soon after that, chat reads "BasketMC was slain by Ancient Guardian." It's the first of many lines like it.
The water is full of surprises. There are charged creepers hiding in leaf blocks, guardian spawners, and a ghast spawner, which somebody points out is not supposed to work underwater. Arrows barely travel. One player's wand doesn't work well down here either, and their bow can't hit anything. The answer to the guardians is line of sight: block it, and they stop beaming you.
The pit
The goal is at the bottom, under a sign, past more creeper spawners and an ancient guardian spawner. Someone dives for it, takes out a spawner "before I died," and dies. A chest at the bottom holds a rare Soul Crusher, a stone axe with Sharpness 8. "I hate this place." "Yeah, this is maybe the worst place ever."
Death piles
From here the episode turns into a salvage operation. JJBonsig, WonderSim and dash10138 all die to Ancient Guardians, and by the end of the episode Basket has died at least six times. Death piles collect at the bottom of the pit, and nobody is sure whose stuff is whose. Players refresh each other's despawn timers, stash gear in a chest, and hand around spare helmets and boots. "This is two days in a row of just crushing deaths."
JJBonsig comes back with nothing on and no weapon. Others pass down a hat, shoes, blocks, an axe, torches, a sword and some chicken. The one loss that matters is a blue shield, but someone mentions that lost quest items can be bought again as replicas now. Another tip: shields do block the guardian beams, a little.
The pressure plate
After a few more creeper spawners are blocked off, the way up is a regular dungeon with no water. A strong bow guy waits in the corner. Then there's a pressure plate. "That's the end. It's as simple as that."
Chat reads "BasketMC has defeated the Magenta Wool Dungeon!" and the screen fills with magenta sparkles. JJBonsig, WonderSim and dash10138 get the same line. Basket levels up and gets a skill point.
While they head to the loot room, talk turns to Minecraft 1.13, the aquatic update, and how new water physics might make this dungeon easier to swim through.
The loot room
The rule is that the loot in your own room is yours. There's another Soul Crusher, 13.5 attack damage, and a spare axe gets offered to the one player who actually uses axes. Basket's chest has Plaguebringer, a rare green leather chestplate with Mending and Thorns IV. The worry is that Thorns wears it out faster than Mending can fix it, and with no regen on it, one of them says they probably won't ever use it. A Totem of Undying turns up too.
And there's the wool itself, a Magenta Wool block, marked as a monument block.
"Loot room, done." That's where this video, and the Monumenta playlist, ends.