Is Excalibur: Morgana's Revenge free?

Yes. Excalibur: Morgana's Revenge is released under Freeware / Community and costs nothing. Neither it nor alephone has any paid content.

What do I need to run Excalibur: Morgana's Revenge?

alephone, which is itself free. Excalibur: Morgana's Revenge is an add-on and will not run standalone.

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

Roughly 180 MB, on top of the base alephone installation.

Yes. The source is published at github.com/Aleph-One-Marathon/emr under Freeware / Community.

Excalibur: Morgana's Revenge — Free alephone Mod · PlayBound