[Update #2]: A closed beta version of Dota Underlords has now been released for Dota 2 - The International 2019 Battle Pass owners.
[Update]: It would appear that Dota Underlords will be Valve's version of the highly popular Dota Auto Chess mod.
It's been known for a while now that Valve is working on multiple new games, some of which are supposed to be VR exclusives that would showcase what the newly announced Valve Index could do. While the exact nature of these games still remains a mystery, you might be interested to hear that Valve has now filed a trademark for one of them - Dota Underlords.
What exactly Dota Underlords is about, or if it's even a standalone game, nobody currently knows. It is entirely possible it could be an extension of Dota 2, perhaps even tie in with the recent Battle Pass and it's new event, though until Valve releases an official statement I'd take all of this as pure speculation.
Speaking of speculation, I'd say that there is a good chance for Dota Underlords to be one of two things: a VR-exclusive game that leverages the Dota name to get as many people as possible to try the Valve Index, or a more polished spin-off of the Dota Auto Chess map.
While the latter is a bit of a pipe dream given that the Auto Chess team has already released their own mobile version, I wouldn't put it past Valve to pick them up and have them make a standalone PC version as well. After all, Valve has made a name for themselves by picking up talented modders and giving them the resources needed to make fully-fledged games, so why not do the same thing again?
Whatever the case may be, I'll make sure to let you know once Valve makes some sort of official announcement. Until then, however, all we can really do is wait and hope the big reveal won't be coming via Valve Timetm