The Royal Pools make the Castellum look pretty simple and tame by comparison. This is your first hard fight of the raid, with lots of mechanics that require each member of your Fireteam to take on a specific role.
The room is a big square, and in each of the four corners are the four emblems you saw in the Castellum: Crossed axes, sun, chalice and beast. Each is set in the middle of a pool of corrosive purple water that will injure you if you stand in it, with a big chain in front of it.
In the center of the room is another emblem, and three sets of chains, with one in the back and two in the corners. These three sets of chains in the center room are your true goal for this room. Eventually, each of the three sets will raise three cisterns out of the water in the room, and you’ll need to shoot them. More on that later.
To raise those chains, you’ll need to send four players to stand on the emblems in the corners of the room, one for each emblem. Spawning on each emblem is an orb that, when you touch it, conveys a buff that protects you from the corrosive water.
For each of these four players, standing on the emblem slowly raises the chains in each of the corners of the room. You need to raise the chains until they lock into place, which will raise the cisterns in the center room. It’s a slow process, and you’ll be fighting swarms of enemies while you do it. For the four players on the emblems, however, the big problem is the Bathers.
Climbing out of the chain pit in each corner are huge, blade-wielding enemies called Bathers. They need to be dispatched quickly, because they’re tough to bring down. The best way to do that is with Power weapons like grenade launchers (and our team found swords to be extremely effective). Don’t worry about dumping power weapon ammo on Bathers, because each one will routinely drop power weapon ammunition to resupply you. In fact, keeping your ammo up throughout the raid with smart grabs is essential.
As you stand on your emblem, the buff protecting you from the damaging water will wear out. This is where the other two players on your Fireteam come in. The only place to recharge the buff is on the center emblem, where it continually respawns. You’ll need one of the last two guardians who are hanging out in the center of the room to grab the buff and then head to your emblem to replace you and continue raising the chain.
The pools on one end of the room are at a higher elevation than at the other, and it’s at this higher elevation that the Bathers come out first. So on each side of the room, you’re creating a rotation in which there’s always a player restoring their buff and heading to an emblem to replace one of the players whose buff is about to run out.
It’ll look like this.
The two players in the center of the room will want to pick one side (left or right), grab the buff from the center emblem, and then first head to the higher elevation pool to replace that player. The player leaving the higher pool then grabs the water protection buff from the center room, clearing whatever enemies they can along the way, before running to the lower pool and replacing the player there (and also helping with their Bather, who will have arrived by then). The player at the lower pool then heads to the center, rebuffs, and replaces the player at the upper pool. Continue this rotation until the team has killed three Bathers at each pool; if you’ve kept someone on the emblem the entire time, all the chains should lock into place. Otherwise, you’ll have to keep trying, and endure more Bathers.
With the chains locked, everyone should head to the center room and clear out additional enemies, before four players step onto the center emblem, where the water protection buff is waiting. Working as a team, those four should focus their fire on a single cistern to take it out (the best way to do this is to pick the order to take down cisterns ahead of time so that everyone knows where to shoot and when). You should be able to take down one set of three cisterns before your water protection buff runs out. When that happens, quickly get out of the water to avoid getting too hurt.
The last two players who aren’t shooting the cisterns are on enemy-killing duty. Bathers will spawn in the room to attack the people in the center of the room, and occasionally those floating-orb Projection guys will pop up as well. The two players who aren’t shooting cisterns have to keep these enemies at bay to protect the folks in the water.