Ink is a special effect in Super Smash Bros. Ultimate that is used only by Inkling in most of its attacks. When opponents are covered in ink, they take additional damage from attacks from other fighters, thus allowing them to be KO'd faster, speeding up gameplay.
Inkling's attacks that use ink
Origin
Ink is a major aesthetic and tool in the Splatoon series, being the embodiment of the Inklings in Inkopolis, and the basis for their weapons. For the Inklings, it is their major form of transport, since they can swim in it, and defense, since they can use it as a projectile, and that an Inkling of different color cannot step in that Inkling's ink, which can be lethal to them. Ink is also used in the annual Turf Wars, where competing teams of Inklings try to cover as much area as possible in their team colored ink.