Link using Cryonis to create an ice block
Build ice pillars
Cryonis is a Rune ability for the Sheikah Slate obtained in Breath of the Wild. It allows one to freeze water into a large, cuboid block on command.
Breath of the Wild
The Cryonis Rune is obtained in the Keh Namut Shrine while exploring the Great Plateau. It can be used to create up to three frozen pillars at a time. These pillars can only be created where there is existing water, and cannot be made out of nothing. Pillars can be created vertically or horizontally, depending on how the water is flowing. If Link creates a fourth pillar while three pillars already exist, the first pillar created (in chronological order) will disappear. Like Magnesis, Cryonis is inexhaustible and cannot be upgraded at the Hateno Ancient Tech Lab.
Link is able to climb these pillars, allowing him to reach new areas that he could not before. They can also be used as block for stopping doors that open and close rapidly, as well as raising gates that could not otherwise be opened. Finally, Cryonis will allow Link to raise certain chests and objects that are floating on top of the water.
The Cryonis ability's power is not limited to ice objects that Link has created; it can also be used to destroy ice created by enemies. This is particularly useful in the fight against the Divine Beast Vah Ruta, and against Waterblight Ganon inside. However, it's ineffective against natural-occurring ice blocks, which have to be melted with fire.
Aside from the aspects mentioned before, Cryonis ice pillars can be destroyed via remote bombs, stricken by ancient weaponry and being shot with Guardian beams, as well as Link swimming underneath them. The Cryonis ice pillars also are susceptible to windy conditions while they build up, as they may migrate by being windswept, though the movement stops once the pillar is complete.
Cryonis cannot be used in thermal waters.
Zelda uses Magnesis with Cryonis blocks behind her in Hyrule Warriors: Age of Calamity