Killer Instinct Season 3's launch trailer recaps all the stuff coming to the game today, but it also offers news for what's coming later. That is Shadow Lords, a single-player mode.
Shadow Lords, according to its official announcement, is a kind of campaign mode in which the player picks a main fighter and then recruits a team of combatants to stop bad guy Gargos. Crafting, of all things, is mentioned, which sounds like a variant on what should be a lot of unlockables. Characters added to the game after Shadow Lords' launch, which isn't specified other than "later this season," will be included in the mode as well.
Other news in the trailer includes a reminder that four more fighters are coming out every month between now and July, beginning with Mira in April and Gargos in May. Also, a new multiplayer mode will be added in July, which the announcement said will tie in with Shadow Lords, so it's possible Shadow Lords will be delivered to the game before then or concurrently.
Killer Instinct Season 3 will be the first version of the game on PC, where it will be available exclusively on the Windows Store. It's also out on Xbox One, and the content in this video will be on both platforms.