What happened at the end of the anime Berserk? This question and its answers will contain spoilers.

I just finished watching Berserk (the original, not the 2016 continuation), and was able to follow the plot up until the second to last episode. 


   At that point, the ground turned into faces and almost all of the main characters got killed. Griffith turned into a weird bat-thing. It seemed like Casca was raped, and Guts lost an eye and an arm.


What is this weird scenario? What happened? How does the plot actually end? Why did Griffith transform? Can someone just explain what happened in the last two episodes of the series (the conclusion)?
For you to understand the ending, you need to understand about the Crimson Behelit that Griffith has.

What is a Behelit?


  The purpose of the Behelits is to create an Interstice between the physical realm and the part of the Astral Realm where the God Hand reside. A Behelit belongs to a predestined owner and therefore, no matter the circumtances, will always finds its way to its owner when his desire to escape his current situation is great enough for it to fully activate so the owner can be transformed into an Apostle. From there, the Behelit would gradually come to another to repeat the process. Normally, a Behelit would activate from being exposed to the owner's blood.


The Crimson Behelit


  Griffith's Crimson Behelit, which he obtained from a fortune teller as a youth, was a rare and unique item said to grant one's greatest desire in exchange for the owner's flesh and blood.


After Griffith sees Guts and Casca together and realizes he may lose all the members of the Band of the Hawk, he runs away and falls in a stream. There we see he finds his Behelit among the river stone.

The behelit activates and summons the four members of God Hand.


  Also known as the Egg of the King by both Nosferatu Zodd and the God Hand, it appears once every 216 years when the time of the Eclipse draws near, its owner destined to discard physical form transcend into one of the God Hand. It is not known whether there were five different Crimson Behelits or whether the same came to each of the God Hand's members in turn. 


God Hand then inform Grifith that in order to become one of them, he must sacrifice all the members of the Band present there. Thus, they all are marked with the brand of sacrifice and are killed by demons. Guts and Casca however escape with the help of the Skull Knight. 

Source: Behelit