A few days ago this trademark was filed by Valve.
Dota Underlords. Trademarking a Dota update doesn't make sense... So a new game?
It ties in with their plans for VR games and I doubt they're releasing a completely new Dota 2 flat game as Dota 2 is thriving and they've expressed no interest in creating new flat games.
And then you look at this. It's on the spec page for the Valve Index. [read edit 2 for why this is irrelevant]
Is this absolutely insane speculation or does it all make sense? I don't even know anymore...
Are we getting a Dota VR game?
Thanks to u/Bradllez for finding the original trademark application
EDIT: Whatever it is, seems like they're serious about it. They've even trademarked merchandise for the game.
Another trademark with the name.
Edit 2: Apparently that calibration screen has been used for years, as you can see here. This likely means it is not in fact a hint or easter egg.
"In Dota 2, the Underlord Vrogros is a playable hero and leader of the Abyssal Horde, a nasty bunch of subterranean conquerors. He's a bad dude and wants to swallow up the overworld and enslave everyone. It's a pretty genetic title, so it might have nothing to do with Vrogros, but a game exploring Dota's magma-filled underground realm doesn't sound too far-fetched."
What if they make a spin-off story of this character?
If it's actually a game, it MUST be revealed or even RELEASED at The International Dota competition