Official install.
Battle.net is a community add-on for Hearthstone. Official install. Use the PlayBound launcher to drop the archive into the correct Hearthstone content folder, then enable the mod from the game's mod or content menu. Mods are versioned separately from the base game — match the recommended engine or client release before joining multiplayer, and keep a clean backup of your settings if you experiment with several packs at once. PlayBound only lists projects that publish redistributable archives or clear external download pages. Always read the upstream README for credits, required assets, and known incompatibilities.
Download page
This package isn't one-click yet. Open the official download page in your browser, then place files in the game's mods folder.
Install Hearthstone first if you have not already — Battle.net is an add-on and needs the base game.
The quickest route is the PlayBound Launcher: open Hearthstone, find Battle.net in its mods list and press Install. The launcher downloads the official release and places it in the right folder for you.
Battle.net is distributed through https://hearthstone.blizzard.com. Follow the instructions there — some add-ons install through the base game's own content browser rather than as a file.
Start Hearthstone and enable Battle.net from the in-game mods or add-ons menu. The download is roughly 40 MB.
Yes. Battle.net is released under Community / External hub and costs nothing. Neither it nor Hearthstone has any paid content.
Hearthstone, which is itself free. Battle.net is an add-on and will not run standalone.
The PlayBound Launcher installs it in one click from Hearthstone's mods list. You can also install it by hand — full steps are on this page.
Roughly 40 MB, on top of the base Hearthstone installation.