Goblin slaying
It starts out extra dark to establish that goblins are a very real threat, despite them being low level enemies. Most adventurers don't bother to fight them because of their low level and low reward, leaving them to new adventurers who can't handle them due to a complete lack of experience. Goblin Slayer has a personal grudge against goblins, and continues to hunt them despite being a very high ranking adventurer at this point in his life. Most of the manga has him killing them without too many casualties, if any, but the first chapter/episode is very dark, characters are killed and/or raped, and the threat of that potentially happening again is present throughout (though relatively unlikely with Goblin Slayer around)