Increase Level

is a Dungeon Special available in Dungeon Keeper and Dungeon Keeper 2. As the name suggests, this special increases the level of all creatures under your command by one.
It's a good idea to check if any of your creatures are about to level up. If your creature has nearly reached their next level, then train it to get that level and then use the special, because using this special will put the experience bar back to zero.
It takes more gold and time to reach the higher levels, so this special is more effective if you wait until your creatures have trained a bit first. It is advisable to let your Imps store it in your Library until it is ready for use.