Well, you notice there's a big difference between DK1's Horned Reaper and DK2's Horned Reaper. DK1 he was quite a demon beast and all it was to him was KILL!!! DK2 he seems to think a bit more, and the way you say it seems he is like, divising plans and listening to the higher up doesn't seem like the original actions of the DK1 beast. Besides, if he does divise the plans then... HE STOLE OUR JOBS!!!
Dungeon Keepers plan, not Horny. :P
If you've played my RPG... and even if you had I don't think you'll know what I'm about to say... it was to be revealed later...
The way I see it, the Avatar had pushed away the creatures down to the "Creature Pool" and used a powerful spell to lock them away where they could no longer enter the overworld and wreak havoc as they did previously.
But it was then that the Demon spirits decided to cause the havoc themselves, and took the half physical form known as the Dungeon Hearts. These spirits held power to allow them to surpass that magical barrior placed on the Entrance Portal to prevent creatures from coming up. This is only why a "claimed" Entrance Portal do creatures come up. This is also why they are loyal to the Dungeon Keepers.
The "Portal Max" depends on how powerful the spell on that particular Entrance Portal is. When a Keeper is destroyed, the creatures flee back under into the Entrance Portral to avoid death by the Heroes.
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King Regy is no Avatar, he does not possess magic at all. But I believe he still managed to force the creatures back down to underground as did the Avatar. I suppose his Wizards used the Portal Gem for that very purpose. And I also figure the Avatar's magic still held tightly even though he had fallen, which is why the creatures still use the Entrance Portal obtained only by those of Dungeon Keepers.