Lost Kingdom: The Emperor of the Demon Cave

You play as a Roman-era adventurer exploring the massive underground temple-city built by Emperor Hadrianus in honor of his beloved Antinous. The game is a hack-and-slash action RPG where you delve deep into the labyrinthine dungeons, fighting hordes of monsters and thieves while uncovering the emperor's hidden secrets and tragic fate. Gameplay features real-time combat, equipment upgrades, level progression, and exploration of multiple areas like the Column Corridor and Aven Road. Characters include the player character (no fixed name specified in sources) and various monsters/thieves in the lost regnum. It stands out for its large-scale dungeon crawling, action combat on PSP, and mystery of a legendary emperor's isolation in the Makutsu no Koutei, a classic portable RPG from 2007.