Mount Infinity - Digimon World 1
Mount Infinity is the final dungeon in the game and as you can imagine, it's also the most difficult. To gain access to this dungeon you'll need to increase the Prosperity in File City to 50, you can do this by recruiting a lot of Digimon. Check out my Chapter 1/Chapter 2 Walkthroughs or Recruitable Digimon Guides for more information on this.
Once you've recruited enough Digimon talk with Jijimon to trigger the Airdramon fight in File City and you will unlock Mount Infinity. As for the dungeon itself, it's very linear and the only difficulty will be attrition as you make your way through the dungeon. Stock pile items and come here with a strong Digimon to be successful.
Devimon
Requirement: N/A
Devimon is the first Digimon you'll find in Mount Infinity, he will be blocking your path forward so you must defeat him.
Megadramon
Requirement: N/A
The second Digimon you'll find on this map is Megadramon and he is an optional fight. He is blocking two treasure chests a few floors above Devimon.
Metal Greymon
Requirement: N/A
Much like Devimon before him, Metal Greymon will be blocking your only path forward through this dungeon. Defeating him is required not just for recruiting him but also for progressing to the final boss and beating the game.
Digitamamon
Requirement: Beat the game & save. Reload the file and return to the final boss room in Mount Infinity
Venture back into Mount Infinity, return to the final room where we battled Analogman and you'll find an egg sitting here. Interact with it for a battle, once you win Digitamamon will join your city.