Is Edge of Darkness free?

Yes. Edge of Darkness is released under Free fan mod (see the download page) and costs nothing. Neither it nor STAR WARS™ Knights of the Old Republic™ II: The Sith Lords™ has any paid content.

What do I need to run Edge of Darkness?

STAR WARS™ Knights of the Old Republic™ II: The Sith Lords™, which is itself free. Edge of Darkness is an add-on and will not run standalone.

How do I install Edge of Darkness?

The PlayBound Launcher installs it in one click from STAR WARS™ Knights of the Old Republic™ II: The Sith Lords™'s mods list. You can also install it by hand — full steps are on this page.