There is a 'DLC' that greatly increases difficulty.
It's a mod called 'Epic Encounters' and it can be found in the Steam Workshop and on the Nexus.
The final boss, the void dragon has had its vitality increased a lot compared to when the game was released.
(But in a very cheap way I have to say, the health pool difference is almost non existent between Explorer, Classic and Tactician Mode due to the cheap way it was done.)
