Is Dawn of Darkness free?

Yes. Dawn of Darkness is released under Freeware fan mod and costs nothing. Neither it nor Quake II Enhanced has any paid content.

What do I need to run Dawn of Darkness?

Quake II Enhanced, which is itself free. Dawn of Darkness is an add-on and will not run standalone.

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

Roughly 358 MB, on top of the base Quake II Enhanced installation.

Dawn of Darkness — Free Quake II Enhanced Mod · PlayBound