Valve have teased what they're calling 'The Big Update' to release in early October, with the final release due not long after that for the first official season.
The news comes from the first of two smaller updates released over the last few days, all update notes can be seen here. What Valve said they will be doing is adding in 2 playable Underlords, the Duos team mode, 6 new Heroes, 3 new Alliances and an updated user interface. That will come sometime in the first part of October, with the "final stop" (the 1.0 release) to come shortly after with 2 more Underlords, the proper Battle Pass, the City Crawl and the start of the first season.
Giving another teaser, they showed a new main menu screen:
As usual though, details on exactly what all this means aren't too clear. It's good to see a team at Valve communicate a huge amount more, since Valve have historically not liked talking much. The key details we're missing are what are the Underlords and what does your choice pick actually do? Additionally, this City Crawl has piqued my interest which again they've not said what it is. Some sort of PvE progress perhaps? Will let you know when I know.
Find Dota Underlords free to play on Steam.