Twinmold

Giant Masked Insect: Twinmold (大型仮面虫ツインモルド) is the boss of the Stone Tower Temple, the fourth and final dungeon in The Legend of Zelda: Majora's Mask. The boss battle is fought in what appears to be a huge, enclosed desert.

The Twinmold boss consists of two large, segmented worm-like creatures. On their heads are a pair of large pincers and three large eyes. On their tails is what appears to be a stinger. The difference in color of the two worms may indicate a difference in gender, like with the Gyorg Pair. It is possible that this is the largest boss in the series.

Story
After exploring the Stone Tower Temple, Link unlocks the door to the boss room with a Boss Key. This is the same room where he fought the Garo Master, but instead of a hole in the floor with sky below, he finds a portal in the floor. Jumping into it, Link is transported to a huge desert filled with stone monuments, some with depictions of Majora's Mask carved into them. Shortly thereafter, the monstrous Twinmold appears, burrowing out from the ground.

Battle
The easiest and most apparent way for Link to defeat Twinmold is to equip the Giant's Mask found earlier in the dungeon. Unlike other bosses, this boss is very hard to defeat using the Fierce Deity's Mask. The Giant's Mask will increase Link's size until he is the size of the worms. He can then strike their vulnerable heads or tails to damage them. Link is not able to Z-target Twinmold when he is this size, possibly because Tatl, who allows Link to Z-target, does not grow in size with Link. After defeating one half of Twinmold, its body explodes, leaving its head embedded in the ground. After both Twinmold halves have been defeated, the battle ends.

An alternate way to defeat Twinmold is to use arrows or bombs when the heads or tails come down close to to the ground.

Interestingly, if Link strays too far from the center of the desert, he falls through the sand and is forced to fight his way back to the boss lair and repeat the battle.

Video
Battle with Twinmold