The Final Ascension was a major event on Scadrial and in the Cosmere as a whole. It marked the the merging of Ruin and Preservation into a single Shard, Harmony.
Leras died having expended all his energy after his final conversation with Elend, leaving Preservation without a holder. It would have gone straight to Vin, except for the fact that she was still wearing her earring, which was hemalurgically charged, acting as a repulsive factor.
During the battle with the Steel Inquisitors, Marsh tore out Vin's earring, leading to Preservation, and the mists, being absorbed by Vin. As Preservation, almost all her actions are prevented by Ruin, held by Ati. As such, she has to watch as Elend dies in the Battle of Hathsin. Following that, she kills Ati, and herself in the process, leaving both Ruin and Preservation unheld.
The Final Ascension
After Ati and Vin had perished, both Ruin and Preservation were unheld. Sazed was able to pick up both of the two Shards. He became a Shardholder, and the shards, merging together again under a single shardholder, became known as Harmony.
The primary repercussion of the Final Ascension was the reforming of Scadrial. The two shards, working in harmony together, were able to reform Scadrial back to a relatively similar world to what it was like before the Lord Ruler's Ascension. Sazed had the ability to do this better than most, because of his duties as a Keeper. Scadrial was formed into the Alloy of Law Era world, with the Elendel Basin replacing the area where Hathsin once was.