After you go through Spool Swamp, you get the Power of Summer added to the Rod of Seasons. Use the Rod of Seasons on the tree stump north of where you used the floodgate key. Go north and you can climb the vines to enter Poison Moth's Lair.
Get a Key
Go north from the dungeon entrance. In the next room, use your shield to bump into the Spiny Beetles and flip them over. Hit them with your sword while they are upside-down. After you get rid of them, the north door will open. You don't have to go to the west from this room, because it's just an owl statue that tells you to use your shield against the spiny beetles. Go north from the spiny beetle room.
Get past the moldorms and exit east. Keep going east, getting past more spiny beetles. Go east once again. There are wallmasters in this room. There is a switch at the bottom of the room that won't stay pressed, so push a pot from the right side of the room onto it, then go through the door.
There are Arm-Mimics in this room that move the same way as you do except in the opposite direction. There is also a Whisp floating around in here. Ignore the stairs in this room for a moment and go west. Ignore the switch and go south. Use the power bracelet to push the rollers out of the way and get the key from the chest. To get out of this room, go to the right side and push the top-right block up.
Go back north, then east. Go up the stairs in this room. Kill the Pols Voices with bombs. Push the northmost block up, then continue north. Kill the moldorms and open the chest that appears to get bombs.
Get the Map
Place a bomb against the north wall where the tiles are different. Go through and open the chest to get the dungeon map.
Go south from where you got the bombs and push the block next to the rightmost block down. Go to the south wall and push the third from the bottom block left, then the one below you down. Exit west.
Push the leftmost grey statue down. Push the leftmost red statue down. Push the middle grey statue to the right. Now push the lower red statue to the right until it is below the rightmost grey statue. Push the remaining statues down except for the rightmost grey one. Then push the rightmost grey one to the space where there is no statue, and push it down. The door to the left will open.
Go west until you can't go west anymore. Then go north until you reach a locked door. Use your shield to bump the hardhat beetles into the holes. Go down the stairs and go northwest to get rupees from the chest. Go back upstairs.
Get the Roc's Feather
If you fall, just go east and walk up the stairs to get back to this room. Go through the locked door. Use pegasus seeds to get across the quicksand, being careful of the spinning blade trap. Kill the the Arm-Mimics to reveal a chest, which contains the Roc's Feather. When it is equipped, you can jump.
Get the Compass
Go south from the room where you got the Roc's Feather. Go to the left side of the room and jump onto the moving platform, then fall down the hole on the left side of the room. Go down the stairs here. Use the Roc's Feather to make your way to the ladder on the right side of this area. You end up in a room with an owl statue. Jump on the trampoline in the upper-left corner to reach the upper floor, where there is a chest that contains the Compass.
Get a Second Key
Go back down the hole. Push the trampoline to the pink tile at the bottom-right and jump onto it. On the upper floor, go east. Push the first roller to the right, then jump over the spikes above you. Push the second roller to the right, then jump over the spikes below. Jump over the lower spikes and push the third roller to the right. Stand in the gap above and exit right. Go down the stairs.
Push the trampoline left all the way, then down twice, then right all the way, then onto the orange tile on the right side of the room, then jump onto it. Get the key from the chest.
Get a Gasha Seed
Drop down the hole then go down the stairs. Jump onto the first moving platform when it's below you, then do the same with the second moving platform. Wait for the second moving platform to be at a higher level than the ledge on the left, then jump left onto the ledge and exit west. If you have trouble, use a Pegasus Seed to get to the ledge.
In the next room, you can either drop to the bottom level and use the moving platform to get to the left side, being careful not to let the Thwomp fall on you, or jump over the Thwomps to get to the ladder on the left. Go up.
Use the Roc's Feather to ride the moving platform. Go east, then south, then west. Use the Roc's Feather to jump over the switch. (Pressing the switch makes the tiles fall.) Go up the stairs and use the Roc's Feather to jump over the blade trap. Go east, then north, then west to get a Gasha Seed from the chest.
Reach the Mini-Boss
Go back the way you came, to the room with the falling tiles. Unlock the door on the west.
When an Omuai comes out of the water, use the Power Bracelet to pull it out and throw it. Hit it with your sword while it's jumping around. After they're all gone, go up the stairs in the bottom-right corner of the room.
Get the Boss Key
Go west, then up the stairs and west. Ignore the ledge that you pass. Continue west, then jump over the gap (just walk across to jump automatically) and fall into the hole. Be careful of the huge blade trap. Push one of the blocks into the large hole near the bottom, then jump over. Get the Boss Key from the chest.
Reach the Boss
Jump back over a hole from where you got the boss key, then go to the southeast of the room, push the bottommost block to the right, and jump down the ledge. Go east, then south. Use the warp point to get back to the mini-boss room. Use the stairs at the bottom right.
Go west, then jump over the first gap that you pass. Follow the path to the boss door.
Jump to a middle platform as soon as you can. Mothula will shoot small moths at you. Get rid of these with your sword. Mothula then spins rapidly around the room. Stay in the middle while it does this. It will stop on one of the middle platforms and shoot fireballs. Jump to the middle and hit Mothula then. If you fall down below, jump on the trampoline to get back up.
Get the Essence
Get the heart container and go down the stairs that appear. Get the Bright Sun. Outside, the Maku Tree says he senses an Essence behind the waterfall on the mountain north of Sunken City.