Defeat the skeletons in the two hallways (J7-8) and (J8-9). The two hallways are each lined with 6 Skeletons. The Skeletons don't aggro or link, and are weak, but if anyone attacks Keremet they will rush to aid him.
Defeat all 12 Skeletons first, then pass through one of the doors at the end of the hall (K-8). Once they are defeated you have about 30 minutes before they respawn.
If you don't want to, or if you forget, these 12 Skeletons are so weak that two Stonega II blasts from 2 BLM (or Banishga II from a WHM) will completely destroy them, and do some healthy damage to the NM.
If you are doing this with only 2 people, farm one Coral Crest Key for player 1, and one Sealion Crest Key for player 2. After opening the lock the first time, farm another Coral Crest Key for player 1. This is the only way to open the door.
Old Professor Mariselle
All quest steps below this point are mandatory to complete the mission.
Unlock the Door
Head to (H-7) (second map) in the Sacrarium and find a door with two keyholes
If you have the Temple Knight key, simply click either keyhole and proceed through the door, and then skip to the Cutscene section below.
Both keys must be traded. Coral Crest Key first, Sealion Crest Key second. When you get the text "<Player> is holding the lock open...", that is when the Sealion Crest Key is traded. When the Coral Crest Key holder gets the message "<Player>'s hand has grown numb!" then it's too late to trade the Sealion Crest Key. make sure that whoever makes the first trade is ready to give a text signal i.e. "go" or "now" when you get the first text about the lock being opened.
It's highly recommended to use Poison Potions to protect against his Sleepga spam.
All three NMs teleport around the room every ~20-30 seconds. Mariselle "despawns" first, along with his two pupils, and then respawn randomly somewhere in the room (the pupils also spawn randomly, sometimes all three are far apart after a teleport). Even if pulled outside the room, it will respawn somewhere in the room again.
Proceed back to the locked door and open it with your second Coral Crest Key and the original Sealion Crest Key, using the same method as before. Alternatively, simply have someone operate the switch on the other side.
Click on the Wooden Gate again to end the mission.
Once the cutscene is over, you will end up on the other side of the Wooden Gate, unable to open it from that side.
Now that you have learned the Wyrmking's plan, you must travel to the fourth mothercrystal. Nag'molada has informed you where the stone lies. However, you will need the amulet you lent to Prishe before you are to make your way there.
SPOILER WARNING: Click here for additional details about this story line.