For centuries a barbarian tribe [need tribe name] has protected the world from the evil of the cave, going out of their way to ensure travelers do not enter the cave and the monsters do not leave. Recently, activity spewing from the cave's mouth has increased and the barbarians sent out runners to identify heroes who can help investigate the cause of the increased activity.
Unbeknownst to the barbarians they have a new neighbor. His name is Don Jean Me'Stiere. He's a gnome of small stature but grand plans. He moved in a few months prior and has been busy preparing the greatest dungeon the world has ever seen.
A mage of great ability, he spent many years travelling the [planes of existence/world] hunting down the greatest challenges possible. He collected knowledge of the dungeons, recorded their locations and ranked them on difficulty. After many years he returned to his home location and starting shopping for a place to begin his master-plan.
He had heard of the great cave throughout his child-hood. Guarded by a ferocious barbarian tribe and home to unknown monstrosities... it was perfect!
Using his knowledge of the arcane arts he created portals connecting his favorite dungeons together. He sorted them in order of challenge level and when he was ready for 'guests' he started routing some monsters down the valley to attack the barbarian village...
That would be sure to draw some attention.
Keen to have as much fun as possible with this and having a UFO turn up in my fantasy world could be damn bloody interesting. 
