Ohm's Chess is standard chess except that each non-pawn piece is also a Dota hero that has a special ability that players can use only once throughout the game. Examples of heroes and their abilities include - 

1. LICH: Sacrifice one of his team's pawns.

2. NATURE'S PROPHET: Teleport to anywhere on the board.

3. PUDGE: Hook (pull) a piece to himself.

4. AXE: Force the enemy to capture a piece.

For a full list, checkout the heroes page

Ohm's Chess is a custom game in Dota 2 (free to play) which is distributed via Steam, so to get started - 

1. Install Steam: https://store.steampowered.com/about/ and sign-in to it

2. Install Dota 2 via Steam: https://store.steampowered.com/app/570/Dota_2/

3. In Dota, navigate to the "Arcade" tab from the top menu

4. Search for "Ohm's Chess" using the search bar on the right

5. Click the install button on the Ohm's Chess page, and you are good to go!