Valve has applied for a trademark for Dota Underlords; it's definitely a game, but there's not much more information available beyond that. The application was filed earlier this month and categorises Dota Underlords under computer game software, electronic game software and video game software.
With the lacklustre reception to Artifact and Valve's decision to pause the card game spin-off, it's a surprise to see them moving onto a new game, if that's what it is. But it could equally be related to Artifact. In March, Valve's Jeep Barnett told players that the team would be putting their heads down and fixing the game instead of releasing updates, so maybe Underlords is Artifact 2.0.
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.
I'm rather partial to Andy Kelly's prediction, which is that it will be a Diablo-style dungeon crawler. Certainly, the description of the Abyssal Horde's stomping grounds sounds like a fertile setting for an ARPG.
Got any convincing theories?
