The lighthouse is a short dungeon with one only boss in it. As you enter, use the magnes glyph and hold left to be sent flying to the other side without getting hit by the spikes. Continue right to the next room. Then open the boss door and head inside. At first, there doesn't seem to be no boss when you enter the room. Keep climbing the platforms up until the boss makes his appearance.
Strategy: This boss is much different than most bosses in the game. You can't kill it by reducing his hp to 0. You have to use the elevator at the top and squish him to death. And to do that, you have to let the boss destroy three ceilings on your way up.
When the battle starts. climb all the way to the first ceiling. and stay at the top platform until the boss stops climbing. once he's done climbing, climb down to the bottom platform and use your Macir glyph for some damage. Avoid his attacks while trying to attack him and the best spot to avoid the claw attack is you must go to the right side. Once enough damage his down, he will turn red, but first he will move his claw to the bottom left platform. Jump up to avoid it. Once he turns red, duck down to the bottom left platform to dodge his claw attack. After the boss attacks the ceiling twice, move up to the middle of the top platform and backdash when he's just about to strike, you should avoid it. The reason while is that his claw will recoil and hit you even while you're in the bottom left platform. After that he'll use a bubble attack. After that, repeat until the ceiling is broken.
Now keep climbing until you reach the second ceiling. Now you have to use magnes to avoid his attacks. After he turns red, stay in the bottom left platform. Unlike the last ceiling, the ceiling needs to be attacked twice on the left side to break.
After the second ceiling breaks, keep heading up. You'll have to use Magnes to reach the top. Now the right side has no platforms. Using Magnes is crucial here for dodging his attacks. Once he turns red again, head to the top left platform and get very close to the wall as you can. His claw won't hit you. The ceiling needs 3 hits from the left side to break.
After the third ceiling is broken. Continue up to the top. The elevator is there. Get inside of it as press down. The elevator will come down on Brachyura and kill him at the end. Now use the elevator to go back up to the top. Choose any side you may want (it doesn't matter since they are just the same rooms connecting to one room. you'll might want to go through both sides for the map completion for this dungeon.) At the middle is the Luminatio Glyph. Absorb it. The light from the lighthouse will disappear since you absorbed its glyph. Head back to the elevator room and use it to go down to the bottom of the floor. Head right.
Saving the third villager: Eugen
Go down. You'll see another villager frozen in a large crystal. Absorb the Torpor glyph. Once you freed Eugen and he leaves back to Wygol Village, go left. Another yellow chest. Open it for a Serpent Scale. This relic allows you to go underwater. Which means we can now go through Kalidus Channel the whole way now. Exit left.
At the travel map, Kalidus Channel has the NEW icon below it, meaning a new area in it can now be access. Return to Wygol village, save, stock up on some potions, and head to the new area in Kalidus Channel.