Berserk
Berserk is a manga written and illustrated by Miura Kentaro. It has seen one anime adaptation of the same name.
The series follows the adventures of the protagonist Guts, who does battle against the forces of darkness with a really, REALLY big sword. In fact, it's probably the only anime/manga in existence where the other characters actually notice that someone's sword is unrealistically large, often pointing out "Fuck me, that's a big swoOH GOD IT CUT ME IN HALF BLARGH". It is, in fact, so large that it cannot actually be used while mounted because swinging it makes the horse fall over. Guts himself can only use it by virtue of training with oversized swords since he was a small child, in combination with his titanic (and possibly supernatural) strength.
Chronologically, the story's beginnings are simple gritty medieval violence, following the exploits of a band of mercenaries as they make a name for themselves in the world and acquire power and glory. It swiftly descends into oh-god-what-the-fuck horror when evil deities and their monstrous "apostles" get in on the action, and via a roundabout series of events Guts ends up hunting down his old companion now ascended to godhood, determined to get revenge for his betrayal.
The series has a massive following both in Japan and in the west and has been running for almost two decades. The author, Miura, is well known for taking extended breaks from production, and given his advancing age and his recent statement that he considers the series about one-third complete, has led to fears by many that he will kick the bucket long before he actually finishes the series.