Hypothesis:
The three lords before ascension, fought this great monster. Perhaps the ascension was done to seal it or a direct result of it. So we got to see their past forms and will probably be told this story about them. The lords that remained, maintained the seal perhaps? Maybe that was their reason for leaving behind such powerful evil counter parts to begin with.
Recall that the lords had a piece of the god mask in their possession. If Zobek was telling the truth, which seems very plausible, the lords each took a third of this power and never cooperated with one another, then perhaps the masks, being in the pillars of light, were the devices which kept this creature sealed in the castle. They never cooperated in joining them together because that would release this evil monster. Zobek being Death itself however has no concern for what happens in this world, since he (possibly) cannot die either way. The other two are very mortal as we've seen and have MUCH more to fear about losing the mask pieces and releasing such a monster. It would stand to reason that this monster is more powerful than all three lords combined, putting a good deal of fear into not wanting it unsealed. This would also make more sense in the flow of gameplay and story, to have to fight something more powerful than any of the lords, and perhaps even on par or surpassing Satan's power on Earth.
As far as Satan's goals go, this monster wouldn't hold much weight after he would have powered up with the mask, so that clears any sort of conflict of interest and plot holes there. Despite this, I doubt it was Satan's goal to conquer this beast. More so he wanted to conquer God. At that point this sealed beast wouldn't be much of a threat anyway. It would just be an added bonus.
The beast may even be a previous attempt at Satan's bid for ruling the Earth, or challenging God. The mask could have been created to thwart such evil in the first place. One done, it was split into three for protection, and used to hold the beast in a seal. An ironic sense that Satan would come to covet this same mask to try and challenge God himself.
In this way it makes the lords that necessary evil. Protecting the Earth from befalling an even greater one.
Thoughts?