This special’s called the Ink Storm. Throw this device to create a cloud that rains ink down onto the battlefield. The cloud gradually moves away from where the device was thrown.
— Squid Research Lab
Source: Splatoon’s Tumblr.
…Wanna play? Buy a Switch.
And if you’ve already got yours, click on Splatoon 2 for everything you need to know about the game. 😀