This is not a drill, nor a joke. Dota 2 now has a fighting game mode included as part of the recent Crownfall update. Included within the battle pass, it has five characters, dedicated move lists for each fighter, alternative skins, frame data, and more. It's a little cheap sure, and not quite a fully-fledged fighting game, but it's real. Fun too!
This inclusion - titled Sleet Fighter by the way - was a total surprise addition to the game, and has been recieved rather well! Sure, it's purely PvE at the moment, but as far as battle pass rewards go, it's way more interesting than new skins or currency. It's not the first time Dota 2 has included such a mini-mode either. Roughly four years ago battle pass owners were able to play a Dota Underlords-style hero battler.
In terms of other rewards, Dota 2 players get some treasures, candy snacks which can be exchanged for hero sets, couriers, wards and more. There are also Crownfall coins that provide handy discounts for those who collect enough of 'em. All in all, a fairly standard selection of battle pass prizes aside from, you know, the fully functional fighting game.
