Is War of the Ring: Middle-Earth Scenario free?

Yes. War of the Ring: Middle-Earth Scenario is released under Open Source (GPL-3.0) and costs nothing. Neither it nor TripleA has any paid content.

What do I need to run War of the Ring: Middle-Earth Scenario?

TripleA, which is itself free. War of the Ring: Middle-Earth Scenario is an add-on and will not run standalone.

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

Roughly 35 MB, on top of the base TripleA installation.

Yes. The source is published at github.com/triplea-game/triplea under Open Source (GPL-3.0).

War of the Ring: Middle-Earth Scenario — Free TripleA Mod · PlayBound