Technically the game is free to play, but if you want to play via Early Access (and to have all the champions) you'll have to pay. Assuming you didn't already have access to the closed beta, that is. If so, you can keep playing, but will only be able to do so via the Bethesda.net launcher. Yes, it's a little confusing. The game's free to play version will launch "at a later date."
In addition to the commemorative price cut, the arena combat game has two new maps, upgrade bounties, new character skins and a tutorial. Oh, and I'd be remiss if I didn't mention that Doom Guy from developer id's other highly-influential shooter is a playable character now, too. "Fans of Doom can run amok using Doom Slayer's active ability, Berzerk -- smashing enemies into bloody piles of gibs with his bare hands," Bethesda writes.
Less exciting, but perhaps more useful, Doom Guy retains his double jump from last year's reboot. Handy! See you on the battlefield.