The climactic space battle between the Reapers and all others in the galaxy was largely played out through cutscenes in Mass Effect 3, a role-playing game and not a space combat simulator. Well, a team of modders have seen fit to recreate that showdown in Homeworld 2.
Mass Effect Reborn is the work of Phoenix Interactive; it released in a beta earlier this month and should make a full release by early May. The 429MB download is free, available through moddb.com. Mass Effect R works for Homeworld 2 on Linux, Mac OS and Windows PC.