Fans of the manga on which it's based would have seen Berserk's ending coming long before anyone else. That's par for the course in anime, where most series are based on written stories written long in advance. It's doubly true for Berserk, however, as the showrunner's pulled a classic creator-approved, nightmare-inducing bait-and-switch. Which is to say the anime alters a great deal of the medieval action story to make the ending that much more surprising.
In both the manga and the TV series Berserk mostly follows the would-be noble Griffith, and his mercenary pal Guts.The two come together around the dream of making Griffith king. Which slowly gains traction through political shenanigans, and brute force. The latter is provided by the Band of the Hawk -- loads more mercenaries all working for Griffith. Along with Casca, another mercenary buddy of Griffith's who he saved from being raped some time ago. Because if there's one thing Game of Thrones has taught us it's that writers are pretty single-minded about women in medieval fantasy.
And, yes, Berserk is a fantasy. Only much more-so in the manga than the anime. Elements like a fairy sidekick for Guts were extracted from the TV series to make it more down-to-Earth. The Band of the Hawk does get into a scrape with a vampire-like immortal at one point, but it's a minor plot in a sweeping tale of political intrigue and seriously messed up imagery.
This remains the case right up until the last four episodes or so. At which point Griffith and Guts have a falling out. Guts wants to find his own dream (having been inspired by Griffith), and leaves his very-nearly-highness in the lurch. Griffith isn't thrilled with his bestie abandoning him, and the following fight leaves Griffith questioning his self-worth. He tries to (consensually) bury his resulting sorrows in the current king's daughter, but gets caught after the act.
Being none-too-happy with the tryst the king goes The Man in the Iron Mask on Griffith's finely shaped ass. Leaving it much less toned after Griffith gets a good starving in the royal dungeons. Such was his torture at this point that Griffith is rendered too weak to speak his gratitude when Guts heroically returns to save him.
Then this fucking thing wakes up. It turns out a jewel Griffith wore around his neck throughout the series was some kind of Hellraiser puzzle box that summons a load of demons every 216 years. They offer Griffith a managerial position as king of the demons -- thereby fulfilling his royal dream -- if he serves up his friends as a blood sacrifice. Of course he accepts, rather than abandon his ambition for the friends he made along the way. The Band of the Hawk gets eaten, a restored Griffith rapes Casca (sigh), and Guts heads for the hills...
The end, apparently!