Tail Cave is the first dungeon, which you can reach after finding the key in the Mysterious Forest.
Get a Key
Go to the west and raise your shield, then bump into the Hardhat Beetles to knock them into the hole. When they are gone, a key will fall from the ceiling. Take it.
Get the Compass
Go west to the next room. Go to the treasure chest, but be careful because there are Green Zols hiding in the floor around the treasure chest. Also, when you get close to the chest, the torches in the corner will start shooting fireballs at you.
Open the treasure chest to get the Compass.
Get a Second Key
Go back to the first room of the dungeon and go north. Use your shield and sword to knock the Hardhat Beetle into the pit, then step on the button. A chest appears. Open it to get another key.
Get the Map
Go east and kill all the monsters in this room (Stalfos and Keese). After they are all gone, a chest appears. Open it to get the map.
Get a Third Key
From the room where you got the map, step into the tile in the north wall that has a person shape in it. This is a one-way door. Go west and carefully open the chest that the Spark is spinning around. It contains a key.
Get 20 Rupees
Go west from that room and kill the Mini-Moldorm to make a chest appear. Open it to get 20 rupees.
Get the Roc's Feather
Go north from that room and carefully go north. The Trap in the corner will rush toward you, so don't walk into its path. When the Trap is going back toward the wall, walk north and go through the locked door. Alternatively, you can push the block in the room with the Trap. This will block the path of the Trap.
Carefully follow the path, avoiding the spinning Sparks and the tiny red Zols.
In the corner room, push the leftmost block to make the door open. Go through.
Use your shield to knock the Spiny Beetles upside down. While they are upside down, use your sword to attack them. When they are both gone, stairs appear. Go down the stairs.
In the underground path, go toward the left until you reach a ladder that goes up through the ceiling. Climb that ladder to reach the next room.
Follow the path, then carefully go north and let the Traps slide toward you, then wait until they are sliding away and go north to the chest. Open it to get the Roc's Feather. Equip it and press the button to jump.
Get the Nightmare's Key
Go back the way you came, using the Roc's Feather to jump up and grab the flying heart.
In the previous area, find the room where a line of blocks separates the room into a northern half and southern half. This room is north of where you found the map. Go west and then northeast so you can be on the north side of the line of blocks. Then go north to the next room. Use the Roc's Feather to jump over the gap, then use a key on the key block. Go up the stairs and go west to the next room.
Open the chest to get the Nightmare's Key.
Get the Stone Beak
Go back the way you came, to the room that is divided in half by blocks. Go west and then southeast to get to the south side of those blocks. Then go through the locked door in the east wall.
Go north from this room and you will find some creatures with card suits on them. When you hit one, it becomes frozen. Hit the creatures such that all three have the same card suit displayed on them (i.e. all hearts, or all clubs, or all diamonds, or all spades). When all three match, they disappear, and a chest appears. Open that chest to get a Stone Beak. Now you can check on owl statues to get tips for completing the dungeon.
Reach the Mini-Boss
Go south, then use the Roc's Feather to jump over the gap and exit to the east.
Mini-Boss: Rolling Bones
Use the Roc's Feather to jump over the rolling spiky thing, then use your sword to hit Rolling Bones. Repeat the process until Rolling Bones dies. After this, a fairy appears, and a warp spot appears in this room. The warp spot goes back to the first room of the dungeon.
Reach the Nightmare
Go north from Rolling Bones's room. In the next room, avoid the Traps that rush toward you. Ignore the stairs. Go through the Nightmare's Door in the north wall.
Your goal is to hit the Moldorm's tail. Be careful not to let Moldorm hit you, because it can knock you into the pit and you will have to climb up and try again. Raise your shield to avoid getting hurt when Moldorm rushes around the room.
After Moldorm dies, it leaves a Heart Container. Take it to increase your hearts.
Get the Musical Instrument
Go north to the next room. Walk north to get the Full Moon Cello.
Afterward, you receive a hint about where to go next: Goponga Swamp.