According to Flingnab Cumberbun, a rebel leader in the fight against mutant lava beasts, Underground Monsters have been massing in the core of the earth for the last 20 million muzons. That is roughly seven years. Rebel persimmons such as Flingnab and his minions have spotted these strange UM's, as they're called, releasing demons after being killed by a tiny man in white and blue. These demons are released into Saskatchewan. Why Saskatchewan? Everyone knows that it is the center of the universe.
Saskatchewan and Soul Collecting
So what makes the center of the universe, aka Saskatchewan, so conducive to soul collecting? Well i'm here to tell you! Inverse geometry of the seventh paradigm in thermodynamic newtonian particle physics, that's what. In fact there is a well in the exact center of Saskatchewan where all the souls funnel down into the core of the earth in order to feed The Giant Mealworm of Azazel.
Soul Collecting is a tough business. UM's have been known to die from the stress of it. It's not always great to be a giant monster. It's a little known fact that souls taste exactly like chocolate oranges. This of course makes them a high commodity in communist nations, because we all know communists are soul eating bastards.
War For The Center of Earth
When the Rebels discovered what was going on they diverted some of their forces away from the mutant lava beast wars to start combating the UM's. The idea was to defeat the Um's and take their source of trade: Juicy Souls. This would in turn give them a leg up on the mutant lava beasts and complete their dominion over the earth's core. Suffice it to say, this did not work at all. The UM's used kitten huffing to subdue the rebellion, get them to play dig dug and in turn make their souls sweeter for the taking, because everyone knows kitten huffers' souls are ten times better than regular souls.