This is the 3rd addition to Dominion. It adds new Kingdom cards to Dominion and as it is an expansion, you will need Dominion, or Dominion: Intrigue to play the game.

Dominion: Alchemy is an expansion, and can't be played by itself; to play with it, you need Dominion, or a standalone expansion to Dominion (Dominion: Intrigue). Those provide the Basic cards you need to play (Treasure, Victory, and Curse cards), as well as the full rules for setup and gameplay. Dominion: Alchemy can also be combined with any other Dominion expansions you have.