Play Sekaiju no Meikyuu III - Seikai no Raihousha (J) Online

Etrian Odyssey III: The Drowned City came out on the Nintendo DS in 2008, developed by Atlus. It was part of that era where the DS was full of deep, complex games you could get lost in for hours, and this one fit right in with its focus on pure, player-driven exploration.

You create and command a guild of adventurers, choosing their classes and names, then lead them into a massive, unmapped labyrinth. Your main goal is to chart every floor of this dungeon, drawing the walls and marking points of interest directly on the bottom screen's grid map. Combat is turn-based, requiring careful management of your party's front and back rows, and you need to constantly balance your resources against the danger of venturing deeper. The pacing is deliberate, and the difficulty is unapologetic, demanding your full attention. It feels like a true test of your own planning and perseverance.