Is Hyrule Conquest: Revival free?

Yes. Hyrule Conquest: Revival is released under GPL-3.0 and costs nothing. Neither it nor 0 A.D. has any paid content.

What do I need to run Hyrule Conquest: Revival?

0 A.D., which is itself free. Hyrule Conquest: Revival is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from 0 A.D.'s mods list. You can also install it by hand — full steps are on this page.

Roughly 1.6 GB, on top of the base 0 A.D. installation.

Yes. The source is published at github.com/Perzival123/Hyrule-Conquest-Revival under GPL-3.0.

Hyrule Conquest: Revival — Free 0 A.D. Mod · PlayBound