Spoilerific long summary:
Uhm it wasn't well explained but it seems Griffith conceive himself after raping Casca and that's how he returned to the regular world. Then Nosferatu Zod came for him, and after a brief encounter with Guts that amounted to nothing, they parted ways. Then another arc happened (much better than the one currently animated), when Guts got a party member actually useful. Then Dark Souls bosses appeared and beat up Guts despite him getting a magical armor. And then things started to really drag out with a third party kingdom Griffith eventually defeated with his new Dark Souls bosses army and is on road to becoming king of Midland and making a magical kingdom. Guts pretty much forgot his rage towards Griffith and went on a new trip to a place to restore Casca's memories, fought weird elephant monsters, then some mermans and huge octopus, then they arrived to land of witches and fought a burning man.
The story has advanced slowly and IMO it hasn't gotten as good as the Golden Arc. If anything the best aspect is still getting better and evidently the Dark Souls director took a lot from the creatures and monsters that have appeared. But its most style over substance nowadays, it kind of feels Miura is as tired as the fanbase on this story, and despite how long it has been I don't believe the conclusion can live up to how long it has taken to move the plot forward.