Waterblight Ganon

Waterblight Ganon also refered as the Scourge of Divine Beast Vah Ruta is a Boss fought in Breath of the Wild. It is one of the four phantoms created by Calamity Ganon to take control of the Divine Beasts. It is fought either atop the Divine Beast Vah Ruta or in the center of Hyrule Castle if it is not already defeated. Once the four Divine Beasts are freed, the Monk Maz Koshia recreate the fight in Link's mind as part of the Final Trial. The original Waterblight Ganon is responsible for the demise of Princess Mipha 100 years before the events of Breath of the Wild.

Battle
Waterblight Ganon's weak point is its eye and horns. Landing repeated strikes on these with Arrows will knock him to the ground. An Arrow to Waterblight Ganon's eye will also cancel out any attacks that it attempts. Using an Ancient Arrow or Shock Arrow in particular will deal heavier damage to Waterblight Ganon. In the first phase of the battle, Waterblight Ganon will attack Link using its spear. It will attempt to either stab Link, throw the spear at him, or swing the spear in a large circle. Link can use Cryonis to place blocks of ice between himself and Waterblight Ganon. When Waterblight Ganon attacks, it will break its spear on the Cryonis blocks, giving Link time to attack. If Link perform a Flurry Rush, he will be able to land additional hits on the boss. When Waterblight Ganon reaches half of its health, the water level will rise and four platforms will appear. During this phase, Waterblight Ganon will teleport around the area, stopping above one of the platforms. It will then summon ice blocks, which proceed to move towards Link, dealing damage to Link if hit. Link can use Cryonis to destroy these blocks before they hit him. Waterblight Ganon will immediately follow this attack with its spear, usually by throwing it. To inflict damage onto Waterblight Ganon, Link must stand on a different platform and shoot Arrows at it. Once defeated, Link will be rewarded with a Heart Container and Mipha's Grace.

Hyrule Compendium Entry
"This phantom of Ganon attacked the Divine Beast Vah Ruta and was responsible for the demise of the Champion Mipha. It attacks with a spear and ice magic."