Imagine you're an adventurer in a tabletop RPG, like Dungeons & Dragons. Goblins might be the weakest monsters you can face. Experienced heroes soon move on to more powerful enemies. If someone offered them a goblin-killing quest, they might not even bother. Goblins are the size of children. They're ugly and nasty, but also a bit stupid. One goblin on its own shouldn't be a problem. A band of them... okay, yeah, that could be dangerous. That's especially true in their underground tunnels, in the dark, with a few surprises prepared.
The nastiest thing, though, is what'll happen if a goblin catches you. What happens to an overconfident party in ep.1 is extreme enough that it made this the most controversial anime of its season. Theoretically it's not unlike something like Berserk, but that's famously a journey into hell. This one looks like a bright RPG-based light fantasy... and then things turn extraordinarily bad. We're talking goblin rape, while the hero's a monomaniac who kills goblin children. Crunchyroll apologised and put a content warning on all subsequent episodes.
That said, though, the controversy was at least good publicity. The show absolutely dominated in America and also did pretty well in Canada, Latin America, the British Isles and the Nordic countries. It's going to get a new episode being shown in cinemas. Personally, I thought it was a good show and I enjoyed it. The extreme content gets pretty dark, yes, but this is anime. You don't actually see the worst of it (which makes it almost more harrowing), although I could imagine some viewers disliking the fact that the show isn't without regular fanservice too.
Nonetheless the show doesn't duck the long-term psychological effects, with world-famous heroes like Sword Maiden having mental issues even ten years later and being incapable of doing their old jobs. (Oh, and the goblins also blinded her by burning out her eyes with torches.) Even Goblin Slayer himself is a fair candidate for a PTSD diagnosis. He's barely capable of talking or thinking about anything but goblin-slaying. He never relaxes, checks his surroundings so diligently as to seem almost autistic and never takes off his armour (including his helmet!) unless he's got no choice. He also doesn't seem to feel emotions and his idea of a conversation is literal-minded, robotic answers to other people's questions. He's got no interest in social skills, or indeed in anything that doesn't help kill goblins. He's also completely uninterested in romance, friendship and sex, although in fairness he is polite and he'd have no objection to joining a party if their mission was goblin-slaying. Five people working together are more efficient than one, you see.
When he was young, he was the only survivor of a goblin attack. He saw what they did to his sister.
He's a bit like Judge Dredd, actually. Both keep their faces hidden and spend all their lives on semi-heroic violence. What's different about them is that Goblin Slayer's more vulnerable. He's quite a high-level adventurer (silver-ranked), but he's focused entirely on killing goblins and can get smacked around badly by a big opponent. He's not all-conquering and he doesn't always win. He understands when an enemy's too much for him. He'll get his arse handed to him a few times and will get saved by his friends, which gives more emotional weight to the big moments. They're right to worry about him.
He's entertaining, actually, in his eternal straight-man way. However it's his friends who create the show's emotional texture. The goblin-killing episodes are important, obviously, but just as vital to the show are the quieter in-between episodes where people reminisce about their childhoods, try to be nice to each other and just live their ordinary lives. This can be a paradoxically gentle show. The contrast is important. This is a sword-and-sorcery fantasy setting, but it's more domestic than most high fantasy. Goblin Slayer doesn't care about fighting the Demon Lord. He's not trying to save the world. All he wants is to stop goblins from killing everyone in the villages. Thus it's important that he's staying at the farm of a childhood friend who's in love with him, but also living there is an uncle who calls him a dangerous idiot with a death wish. We get to know these people. They don't go on adventures, but they still matter.
Meanwhile his party members will end up including a fifteen-year-old Priestess he saves and a group of three non-humans (dwarf, elf and lizardman). The Priestess is a kind-hearted rookie who shouldn't be hanging around Goblin Slayer, but she cares about him nonetheless and will sometimes scold him for being too Aspergery. As for the others, the elf complains that adventures are supposed to be fun and invents a sort of Geneva Convention to keep Goblin Slayer slightly under control. No fire, no water, no poison. (Explosions later get added to that list.)
I liked the self-awareness of the gods, by the way. They roll dice to decide the world's fate. Six-sided, eight-sided, twenty-sided, etc.
Personally, I liked the show a good deal, including the extreme content. Some people strongly disagree with me, though. This has been a polarising show. Sometimes it's compelling or oddly charming, but it's set in a mercilessly savage world. Heroes can wet themselves in terror. Goblin Slayer himself is quirky and amusing, but also a psychologically damaged obsessive. I'm looking forward to the movie.