If you haven't read up on the Auto Chess phenomenon yet, I'll wait. Ready?
Okay! So you're now aware that DOTA Underlords is Valve's official take on Auto Chess, which is based off of a mod in DOTA 2, which itself is a sequel to a game that was a mod of a Blizzard game. Maybe the above explainer isn't enough. Anyway, Underlords is a thing now, and Valve is pushing it harder than Artifact, which was supposed to get a bigger push after it was neglected for months on end.
As part of that push Valve is trying to iron out all of the kinks of the current beta format, which includes granting players free reign of the first battle pass, dubbed the "Proto Pass." Basically you just log into the game and you'll net a free extra level track, which unlocks various cosmetics like emotes and board types (a popular microtransaction for other Auto Chess games, which changes the look of the arena) alongside of challenges. It's literally a battle pass guys: the things that are in Fortnite and such to get you to keep paying money every few months on top of all of the other stuff.
As contentious as they may be this one is free, so if you choose Underlords as your Auto Chess game, you have a leg up.
Proto Pass [Valve]