MEHEM is a reinterpretation of the endings of Mass Effect 3, which allows Commander Shepard to survive the final mission, among many other changes. It features custom videos and slides, re-engineered gameflow, original as well as re-arranged voice acting, new music, etc.
The single most important mod I have ever encountered. I hesitate to even use the word "mod" because MEHEM is so vital to the Mass Effect Series. I would encourage you instead to consider MEHEM to be an unofficial patch to fix bugs left in by the developer.
I say Bravo! to all who's hard work made this journey to the stars possible. Ad Astra Per Aspera!