Obviously, SPOILERS for Mass Effect 3 AND Mass Effect 3: Leviathan follow.
So the reason I feel Leviathan is one of the greatest DLC packs for the Mass Effect trilogy is because it answers one of the biggest questions in the lore, a question the Catalyst and his little speech tried to allude to vaguely: Where did the Reapers come from? It totally renewed my love for the series because, honestly, the whole debacle with the ending of ME3 and everybody hated the ending and they were clamouring for a change, it just left a bad taste in my mouth. I didn't think the original endings were all that bad. I feel as though people couldn't get past the fact that all their questions weren't answered, and they don't know what happened to their team.
If anybody knows BioWare, they know that there is going to be DLC. I assumed that before ME3 even came out. If someone makes a movie with a cliffhanger ending, nobody gets up in arms and cries, "I need closure now!" You'll just have to wait until the next movie! Is it a little different because all of our choices seemingly had little to no effect on the final outcome? Yes, of course. But still, I felt the whole outcry and "Hold The Line" movement was just kind of... childish. ANYway...
Back to Leviathan.
So these aliens, the Leviathans (even though that's not their proper name), we saw at the depths of the ocean on a far-flung planet. They reveal that they were the first apex race of the galaxy, and that they have the power to control other organic lifeforms. They referred to other races as their "tools" to accomplish whatever they wanted done. It's similar to the concept of Reaper Indoctrination, but less sinister. These Leviathans observed other races and found that they would eventually (read: over countless generations) build for themselves synthetics to carry out their will. Basically, making robots to do all the hard work and make their life easier. The synthetics would sooner or later realize who they were and rise up against their makers (Quarian and the Geth). So these Leviathan, being the haughty god-complex-imbued race that they were, realized that "Tribute does not flow from a dead race." They then built The Intelligence.
This Intelligence was created by the Leviathans and was given one purpose: preserve organic life at all cost. This Intelligence grew and studied the development of organic races, and came up with the idea for the Reapers. Once an organic civilization would reach the apex of their evolution, their kind would be "harvested" and then "preserved" in a Reaper. The Reaper would have the entire essence of a species within itself and would be a kind of monument to that race, thus "preserving" them.
So basically, these guys were the first race in the galaxy to achieve sentience and rise to power. Their actions brought about the Reapers and started this whole cycle of harvesting. Insane.
This just makes me love Mass Effect even more because it adds a whole new dimension! These Leviathans are basically the creators of the Reapers, even if they were made by the Intelligence betraying them. This just makes me so excited at the possibilities of stories about these creatures, before and after the ME trilogy. Crazy.