Story related; cannot be missed.
This is obtained from the “Styxcoin” dungeon (it’s got a yellow entrance) that is available after you complete the first dungeon (once as a tutorial, and again to actually beat it)
If you’re having trouble, there are accessibility options that make the game substantially easier.
After you beat the boss of the dungeon and obtain the relic, the achievement won’t actually pop until you head back up and add it to the fountain in the middle of the room.