Is TFix — NewDark Engine Upgrade free?

Yes. TFix — NewDark Engine Upgrade is released under Open Source / Community and costs nothing. Neither it nor Thief™ Gold has any paid content.

What do I need to run TFix — NewDark Engine Upgrade?

Thief™ Gold, which is itself free. TFix — NewDark Engine Upgrade is an add-on and will not run standalone.

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

Roughly 150 MB, on top of the base Thief™ Gold installation.

TFix — NewDark Engine Upgrade — Free Thief™ Gold Mod · PlayBound