Hydrus

"Thy next foe is... A ruin hidden in the lake...A ripple of thunder lurks underwater..."

- Dormin

The Seventh Colossus, nicknamed Hyrdus, is a major antagonist from the video game Shadow of the Colossus. It is the seventh boss encountered in the game and a vessel for the essence of Dormin.

Past
Hydrus, along with the other sixteen Colossi, were created by Lord Emon to contain the fragmented essence of the defeated evil Dormin. He set them all to wander the Forbidden Lands, destroying anyone that comes their way, so that Dormin could never be resurrected.

Shadow of the Colossus
When a young man named Wander trespasses in the Forbidden Lands and asks the disembodied spirit of Dormin to resurrect his deceased lover Mono, Dormin agrees under the condition that Wander kill all sixteen colossi (tricking Wander into releasing his essence so he may be reborn). Wander agrees, and sets off.

Hydrus is found in a large lake in the northwest. It will be seen swimming around the ruins and diving into the depths. Upon Wander entering the lake, Hyrdus will swim around below the surface until breaching, sticking its electrical spines over the surface in an attempt to shock and kill Wander. Wander must avoid these and grab onto the colossus' tail when it finishes its breach. Hydrus will then try rapidly diving and surfacing in an attempt to throw Wander off and/or shock him with its spikes. Wnder must wait untilit is safem then rush forward and stab weak points underneath each spike to disable their power, making the area safe. After taking out some of the spikes, Hydrus will dive and refuse to resurface, forcing Wander to let go or drown. It will lurk around in the water before making another attempt on Wander's life, breaching again. wander must repeat the process, disabling the rest of the spikes before reaching Hydrus' head. There, he will be able to stab Hydrus' major sigil, though Hydrus will attempt to throw him off. After the sigil is stabbed, Hydrus will collapse and die, releasing the fragmented essence of Dormin contained within its body before sinking into the lake.

This fragmental part of Dormin will bring Wander back to the Shrine of Worship, where he will be prepared to face his next foe. It will later combine with the other fifteen parts to form Dormin, who will posses Wander and later be banished by Lord Emon.

Hydrus' corpse can be seen as the credits roll.

Trivia

 * Hydrus is the only one that Wander is capable of rendering completely harmless; once its three electrical spines have been nullified, Hydrus has no other means of attack and will no longer be able to inflict any damage at all. It will continue to make passes at Wander as though it is still capable of electrocuting him, but since it cannot do this, it merely passes beneath Wander without him taking any damage.