As just announced after the Splatoon 2: World Championship at E3 2018, the Octo Expansion DLC planned for Splatoon 2 releases tomorrow for the Nintendo Switch. It requires Splatoon 2 in order to play.
The Octo Expansion can be pre-purchased now through the Nintendo eShop at $19.99. When purchasing it now, you will receive two special pieces of gear. The Octo Expansion offers 80 missions and a look into the Splatoon universe’s lore. Completing these missions allows you to play as an Octoling in the online multiplayer modes.
Additionally, a series of Octoling amiibo was announced. The set includes the Octoling Boy, the Octoling Girl, and the Octopus form of the Octoling. These figures are due to release in 2018.