Vah Ruta is a mechanical construct in The Legend of Zelda: Breath of the Wild. It is one of the four Divine Beasts. Vah Ruta is made from Ancient Sheikah technology; it takes the form of a giant elephant in the East Reservoir Lake in Lanayru Great Spring. Ganon sent Waterblight Ganon to corrupt it, transforming it into one of the main dungeons. Link must defeat this boss to free Vah Ruta.
One hundred years before the events of Breath of the Wild, Rhoam Bosphoramus Hyrule, his daughter, her Knight Link, along with four other Champions sought to seal Ganon through the use of the Divine Beasts and Guardians. Ganon retaliated by taking control of Vah Ruta and the other Divine Beasts, turning them against the people of Hyrule. Mipha, the Zora Champion who was in charge of controlling Vah Ruta, met her demise by a creation of Ganon known as Waterblight Ganon, who then assumed control of Vah Ruta for the next one hundred years.
During this time, the Zora are desperate for the aid of a Hylian; the weapons used to subdue Vah Ruta, Shock Arrows, are too dangerous for a Zora to touch, let alone use, given their vulnerability to electricity. However, there are some Zora who blame the Hylians for Mipha's death and prefer not to ask for their help, most notably Muzu. Vah Ruta then spent the next century endangering Zora's Domain and everyone who lives outside of it by flooding the East Reservoir Lake. If the dam broke, it would likely send a significant amount of water throughout Hyrule, if not flood it completely.
After Link talks to King Dorephan and Prince Sidon, and puts Muzu's doubts about Link to rest, Sidon and Link meet up at East Reservoir Lake, assuming Link has a sufficient amount of Shock Arrows. If Link does not, he must go up to Ploymus Mountain, steal some Shock Arrows from the resident Lynel, and then travel to East Reservoir Lake and meet up with Sidon.
From there on, as the duo approach Vah Ruta, Link must get rid of the cubes of ice by either using any type of Arrows or he can use the Cryonis Rune as a means to counter the ice cubes. When Sidon climbs up a waterfall on Vah Ruta, Link must be prepared to aim a Shock Arrow at one of the switches on the Divine Beast's sides. Every time this is done successfully, Vah Ruta starts becoming more aggressive with its attacks: adding more ice cubes and eventually ice mines that pursue Link and Sidon. When all four switches have been hit, Link can then proceed inside Vah Ruta, find the terminals of Vah Ruta, and encounter the Scourge of Vah Ruta, Waterblight Ganon.
Once Link frees Vah Ruta from Waterblight Ganon's control, Mipha resumes control of it, gives Link Mipha's Grace, a powerful healing spell that brings Link back from the brink of death, and takes the Divine Beast to the peak of a mountain, where it directs a targeting laser towards Hyrule Castle. If all four Divine Beasts are freed, they will fire powerful beams of energy that reduce Calamity Ganon to half-health.
The name for Vah Ruta, like all four of the Divine Beasts, is based on a sage from a past installment the Legend of Zelda series. The Ruta name is a reference of the name of the sage of water, Ruto, from The Legend of Zelda: Ocarina of Time.