In college I played Nethack. Then I moved to Moria. Now I stick to playing Angband.
Having implemented one monster that attacks the player, I then proceeded to code up multiple monsters. The basic algorithms stayed the same. I just needed to create an array of monsters. Then I converted anything that referenced the once ghost into an iteration over all the ghosts. This basically meant figuring out the index into the ghost array, and then using that index to get the specific ghost values.
Now with multiple ghosts, the game is harder. Eventually I want the player to be able to get some power pellets like Pacman. Then the ghosts should be trivial to kill for a while. However I think the next action is to implement level ups for the player. If the player can kill a lot of ghosts, then the level increases.
Hmm. Maybe the player can level up faster if they first eat the power pellets and kill the ghosts while they are weak. Decisions decisions.