When I say adult I mean that the darkest it goes is certain themes of sexual harassment and themes of abuse, with specific characters. (explaining any more than that would spoil some important stuff). The game isn't graphic in the slightest however and most of that stuff is just implied.
If you are at all interested in picking up the game, it's available for the PS4. It's been out for around 3 years now (wow) so you shouldn't have too much of a problem with price. It's also a critical and commercial success, featuring artwork and graphic design that blows basically every other game out of the water; if that means anything to your reason as to buying the game. If you enjoy playing Fire Emblem than it is most definitely up your alley.
Anyway we should probably stop talking here considering this is a Bionicle Forum site, so let's either move it to DMs or change topic.