From the main entrance, follow the path and head straight north to the main door, which was blocked by some imperial troops on your first visit. If you have never revisited Sacred Rahet after you first time here, you can get the Morsyn artifact knowledge in the top left corner of the main room once you get inside of the mountain. Head west while inside, until you hit the western edge, then north for a scene by a large doorway. This is the entrance to the deeper part of the mountain, allowing you to gain access to the terromite you need.
Once inside, follow the path to the open area, grabbing the glow spot, then continue west. Keep heading in this direction until you come to a split, where you might want to go west if you wish to collect some treasure from a chest. The only problem is that there is poison surrounding it, which will be inflicted on all characters (even the reserves). Open the chest for 4 Panaceas, then follow the path to the west, where it will eventually wrap around with the split you took earlier (where there was poison to the south).
There will be some enemies here, so take them out and continue south to find another chest surrounded by poison. It’s entirely up to you if wish to get it, as it houses the same contents as the previous one (4 Panaceas). Travel west, then north to another split, avoiding the poison here. Go west first to eventually find a lone chest in an open area, without poison this time, which contains 2 White Spritnites. Return to the split, then follow the other path to some more enemies and beyond that, a save point. Yep, you know a boss is coming up, so heal and venture into the next room to fight it.
BOSS - Knockrock
Knockrock can be a challenging boss, simply because it can deal a lot of damage to your characters, but some positioning will help you last a lot longer. First and foremost, Knockrock can use Fossilize to inflict petrification on your characters, which you should remove as soon as possible. If all of your characters are petrified (or a combo of petrified and KO’ed), then it’s a game over; a Panacea or Sherra’s Refresh will get rid of it. Knockrock only really has three attacks that can hurt you during the fight, but two of them are negated by just positioning your characters in the right places.
Mythril Gun hits in front of the boss and does massive damage (somewhere in the range of 300+ if all hits connect, plus a critical or two), but you can avoid it by putting your characters in a square pattern. Damascus also hits in front of the boss, but you can avoid it by doing the same thing as before, so there’s no worries there. The more annoying part of the fight is when the boss uses Shiny and Steel Wall, which defends against magical and physical attacks, respectively. There’s nothing you can do, but wait them out, since you won’t be able to deal any damage. One last move is called Oreikhalkos, which can hit everyone for around 200 damage.
Since positioning your characters is a big thing in this battle, that is the first thing you should do. Have your melee users to the side and ranged users in the bottom corners, which should make it so Mythril Gun and Damascus won’t hit anyone. Other than that, there’s not much else to worry about, apart from the odd Oreikhalkos and healing anyone that gets petrified. As for any weakness, the boss is actually weak to wind, so any attacks with that property will do wonders. Whenever you are about to finish Knockrock, make sure you fully heal, as it will finish with a bang (Divide), possibly inflicting 300+ damage to some characters.
With that boss down, you will gain the following pieces of artifact knowledge, Fairy Wind, Deus Ex, and Akashic. Heal up and inspect the glowing red spot at the northern end of the room to get some Terromite. Of course, things are quite finished here, as a Bobboulder drops down into the room. As Sherra explains, these enemies will explode when you finish them off, so do so without anyone near them. You’ll need to retrace your steps back through the mine, while avoiding the Bobboulders. There will be a counter in the upper right corner of the screen, starting out at 5 and every time someone is hit with an explosion from the Bobboulders, then it will be reduced by one. Should it hit zero, then you will receive a game over.
Once you get out of the terromite mine, return to the elder on the eastern side and speak with him to get a nice weapon for Sherra, the Show Tuner. This weapon has a lot of magic on it, but the main use for it is that it halves the MP cost, in battle, of all of Sherra’s skills. That’s all for Sacred Rahet, so your next stop will be the Lunar Kiln in Graccia.