You will find the NPC that allows you to enter the instances in the temple on -2 levels.
An item and money are required to enter the instances.
You can enter solo or in a group. Each time you pass the difficulty level, a new harder is unlocked. Higher levels increase opponents' HP and damage, but also gives more gold, experience and loot chances.
Difficulties will be unlocked after each defeat of the previous difficulty. Each next one increases the strength of the opponents, but also rewards such as gold and materials for improvement.