Unlock Every Sound
Select the Necromancer for your character in the character-select screen. While playing a game as the Necromancer, press "Enter" on the keyboard to open the chat box. In the provided space, type "soundchaosdebug." All of the voices and effects in the game will play simultaneously, creating a chaotic, never-ending loop of sounds.
Increase Player Numbers
The difficulty of Diablo II's monsters is based on the number of players in the game. When more Necromancers are present on a map, the monsters become tougher and smarter. They also release more gold, items and experience points when they're defeated. Press the "Enter" key to bring up the Necromancer's chat box. In the box, type "/players x." Replace the letter x with any number between one to eight to add more Necromancers to the game.
Easily Defeat Strong Monsters
As the Necromancer, go to the bridge that leads out of Kurast in Act II. You will see a robed character named the "Dark Wanderer." He will teleport away and summon powerful beasts to battle you. Lead the monsters towards the bridge once they charge you. Summon any monsters (excluding a mage) to fight the enemies while you're on the bridge in front of Kurast. Your summoned beasts will fight and defeat the enemies easily. Because these monsters are powerful, they will release an abundance of gold, items and experience points as well.
Access The Secret Cow Level
Defeat the game as the Necromancer on any of the difficulty levels. Restart the game using that same difficulty and visit the Rogue Encampment. Take the Wirt's Leg and a Tome of Town Portal (you'll already have both) into the Horadric Cube. A portal to the Secret Cow Level will appear. Enter the level and gather all of the items and gold. Be careful not to destroy the Cow King, or your future access to the level will be denied.