This is a classic Lord of Destruction feature that is not known to all players.

During a single player game or tcp/ip or open multiplayer game, press Enter and type the following:

/players X

where X is a number from 1 to 8. Press Enter again. The game will confirm your action.

What this does is increase the difficulty of the monsters, as well as their rewards, to correspond with a multiplayer game with X players in it.

Monster life and experience is greatly boosted, monsters are more likely to drop items and their damage is slightly boosted on Terror and Destruction difficulty.