Dungeon Keeper Wiki

Multiply Creatures is a Dungeon Special in Dungeon Keeper that duplicates your creatures. Your whole army will be doubled. When used correctly, this special can change the entire game, often leading to you annihilating anyone who opposes you.

Spawn in as many Imps as you can afford (there is always a gem seam on any level containing the Multiply Creatures special); the Imps do not require any sort of Lair, so you can just plop down a hunk of Imps. 

However, be warned. If you already have a Horned Reaper, prepare yourself for chaos. It can become extremely complicated; if you dare try to train them, you are going to lose a significant quantity of gold, even with said gem seam. Additionally, due to their foul-tempered manner, they will still get angry, and you having multiple enraged Horned Reapers in your dungeon can make things frustrating.

This is very well-used with few creatures, but with lots, a lot of gold will disappear...

Also, don't forget to build a new Lair for all those new monsters.