I didnt mean anything personally--- Honest.
I understand wanting to be sure that you had a medium that works as a vehicle for story telling, before investing lots of energy. I was just pointing out that it is opposite from what I usually do.
Many games I have played with a "Big baddie wants to destroy the world, oh my!" type theme does not really elaborate on the evil entity's goals or ambitions in any real capacity. Maybe i'm just spoiled(?), but I was just wanting to be sure that your primary antagonist was more than just a 2-dimensional boogedy man. (shrug) It would be interesting if you put quirks about its behavior in, for instance-- It has goals besides trying to murderate you into pink paste, after all-- so giving it conflicting goals in that respect as the protagonist, to cause it to present emergent behaviors can greatly increase the gameplay immersion.
Depending on how much this entity has to potentially lose by being forced to hang around by some meddling humans, its outlook on being summoned can be quite different, making its behavior and priorities different. The "weaker" (rather, less well established) demonic entities may strongly desire to stay here, where the native creatures are so weak and easily conquered-- while the arch-demon types with whole worlds already under their thumbs may percieve their summoning entirely differently, and have much at risk by being summoned--- Some may actively rampage to destroy their own altars to return to their dominions as quickly as possible. I was just pointing out that in most prevailing stories about demons, the demon itself has motives and a very poignant opinion about the practice of summoning-- there IS a reason why demons are summoned into a circle into which they are ritually bound; they more often than not resent the summoners who seek to enslave it.
Making each and every "mission" have a different demon, with a different group of power-seeking morons conjuring up something that they shouldn't, each with a different take on the situation-- could well prove very clever. On some levels, the best course of action may be to instead of destroying the altars, instead destroy the ritual binding-- loosing the demon from the cultist's magical compulsions. (Then watching the carnage as the demon itself systematically exterminates the cultists, and destroys the altars used to drag it there.) In other cases, doing that would be very very foolish. Even more enticing, is the prospect for some of these demons to "Know about you" or your organization. If summoning is this frequent, then depending on the outlook of the demon you could be seen as a beneficial cleanup service, or as a potential obstacle or menace. Again, I could easily see lesser demonic entities seeking to gain a slice of already dominated planes of reality by more powerful agencies, "Helpfully" giving cultists some very dangerous instruction manuals, and having a very strong vested interest in having those summonings continue uninterrupted.
EG-- Imagine if we could give some meddling interdimensional aliens a sure fire way to summon glorious leader, Kim Jong Un and his entire cabinet away from north korea-- or really, any other world leadership for that matter. We could cripple the heads of state of opposed nations, just by some simple exploitations. There is a lot of potential for progressive plot development there. The demon being summoned MAY actively contact the protagonist. Such a demon may even give you a list of other cults or summoned demons that need to be sent back home, to avoid something even more diabolical.
There's also, of course, the potential for an otherwise benign demon being what is summoned. The murkiness and innate perversion of forced subjugation that a demonic summoning implies can take on all kinds of "unexpected" colour, perhaps the demon summoned is a frightened child? Perhaps the demon is actually the equivalent of a humanitarian figure that opposes some arch demon's rule? Maybe something thats a darker, twisted shade of those possibilities? The possibilities there are nearly endless.
The protagonist having such very weak protective magic on their own can be used as a very powerful game mechanic device, as forming alliances in such inter-dimensional power plays could offer some unique perks. Calling in favors, so to speak.
I understand that such considerations are a bit premature, when all you really want right now is just a workable game engine. I just think it's healthy to think about how you want this vehicle to drive, when designing the wheels is all.