Splatoon 2 introduced the Octo Expansion for the Splatoon series.
Splatoon 2 launched for Nintendo Switch on July 21, 2017. Its main multiplayer modes continued the series’ ink-based team battles, while its single-player content expanded the setting and characters introduced in the original Splatoon.
The paid Octo Expansion downloadable content was released in 2018. It follows Agent 8, an Octoling who awakens in an underground test facility and must complete a demanding sequence of missions. Completing the expansion can unlock the ability to play as an Octoling in multiplayer.
The expansion is sometimes associated with Splatoon 3 because that later game also features Octolings and a story campaign. However, the Octo Expansion is specifically downloadable content for Splatoon 2, not a separate sequel.