Icestorm allows Amaterasu to summon a gigantic six-armed hexagonal snowflake that rains down destructive icicles, freezing and damaging anything within its vicinity. This ability has certain advantages over Blizzard, but conversely has some drawbacks. In combat, Icestorm can deal massive damage while simultaneously freezing opponents solid, whereas Blizzard could only perform the latter. Depending on the size of the brushstroke, 3, 6 or 9 ink pots will be used. Even though Icestorm consumes more ink than Blizzard, its range of effect is very wide and has a massive damage output, therefore having a high efficiency on ink consumption. On the other hand, Icestorm cannot coalesce floating ice crystal in chasms into floating ice blocks to be safely crossed. Also, if Icestorm is used on the environment, then the distance of the brushstroke's area of usage can be distorted, because the system does not recognize a small drawing as to be used from afar, but rather creates a small shower of icicles. Using Icestorm on far away targets can be troublesome: since the distance of the snowflake cannot be determined by the size of the brushstroke, Amaterasu has to rely on an high up overhead view to draw the technique over the target, but this frequently is mistaken for a snowflake drawn at a high altitude, instead of an Icestorm drawn on a target. Blizzard does not have this problem, because it is a source-target brushstroke, meaning that its target, or the brushstroke's endpoint, is defined, regardless of distance. In conclusion, Icestorm excels over Blizzard in regards of damage, range of effect and ink efficiency, but lacks the precision that the latter possesses.
To use Icestorm, Amaterasu has to draw an X. Also, it can be drawn as a cross formed from a horizontal and vertical line each, creating a square angle, then a diagonal line through the center from any angle.
While Amaterasu was exploring Wawku Shrine on the mountain of Ezofuji to look for the twin demons Lechku & Nechku, she came across the source of the blizzard that had been plaguing the entire Northern Lands: an infernal, phonograph-like contraption that blew out snow and ice from on the mountain to the tundra below. Deactivating the machine, Amaterasu found the source of the blizzard: the Celestial Brush god of ice Itegami manifested in the form of his conch horn, forced by Lechku and Nechku to create ice and send them over the entire Kamui region, as the god had to watch the land wither at his own actions, albeit against his will. Freeing the god, Amaterasu regains the last of her lost Celestial Brush techniques, Blizzard, alongside Icestorm.
Spoilers end here.
Icestorm is the only secret Celestial Brush technique immediately obtained when the primary technique is regained. No other technique is obtained in such way.
Icestorm is the only Secret Celestial Brush technique without a Travel Guide scroll, due to the technique being immediately obtained when Blizzard is regained.
In the final battle with Lechku & Nechku, when Icestorm is used on an already-grounded boss, the demon will be frozen and immobilized for a longer period than natural stunning, therefore more damage can be caused. This also causes additional damage aside from the freezing effect that Blizzard would also deal out.