The Twilit Bloat is a giant Shadow Insect with tentacles emerging from its abdomen that can release electric shockwaves. This larger variant has the unique ability to swim underwater. Once Link obtains fifteen Tears of Light in the Lanayru Province, the Twilit Bloat will appear in Lake Hylia, holding the last Tear of Light.
Wolf Link must wait until it is not bursting with electricity. He must then L-target it and leap onto its lower section, then proceeding to hold the L button and repeatedly press A to bite it. The young hero must repeat these process until it fall into the water, lying on its back. At this point, Link must jump on its belly and use the energy field with Midna's help to attack its six tentacles at the same time. Once they are all destroyed, it will be defeated. This will release the last Tear of Light Link will collect, allowing him to finish returning light to Hyrule.
The Twilit Bloat highly resembles a type of sea crustacean known as a Giant Isopod.
↑"Twilit Bloat ... Disturbing as it may seem, you have to defeat this bloated creature with bite attacks. It will be a true measure of your heroism. And it's the only way to fill the Vessel of Light to restore the spirits! Encountered In: Lanayru Province" (Twilight Princess Prima Official Strategy Guide (Prima Games), pg. 22)