MUSING ON MONOTHEISM
Have you ever looked at the diversity of gods in the typical Dungeons & Dragon setting and wondered what it would be like if the deities weren't so distinct? What if divine authority led back unquestionably to one source, one sublime being beyond mortal comprehension? Although an open-ended polytheism is the status quo of most Dungeons & Dragons games and similarly styled settings, some world-builders find it more compelling to deal with monotheistic or semi-monotheistic settings, where one god or goddess has essentially unchallenged hegemony over the world stage.

Restraint
Of course, if there is an omnipotent busybody working miracles constantly, every time something gets on his nerves, he would be a nigh-insurmountable obstacle to a fun D&D game. An important part of what usually makes a one-deity dynamic work is restraint on the part of the sole or head divinity. A strongly interventionist god can easily paralyze any action and prevent most conventional forms of tension, while a restrained god will make it all the more important and dramatic when he is driven to work (presumed) miracles. One side in any conflict cannot be sure the deity is working for them, against them, or not taking sides. Even priests might be kept in the dark as to whether they are doing what their deity wants if the restraint is handled right.

The appeal in this approach versus the typical game setting arguably lies in the mystique of an exceptionally vast and all-encompassing deity. The characters can use past actions and legends to guess at what this being's ultimate goals are, but they can rarely feel confident about them. For example, in R. A. Salvatore's world of Corona, what few events are probable miracles come with little apparent rhyme or reason, aside from perhaps rewarding the devotion of a particularly thoughtful believer.

Mystery
An interventionist monotheistic god doesn't have to be a bad thing. If the god's goals are carried out in a dramatic manner without the players having knowledge of what the god intends, then the intervention functions more like a fickle force of nature than a domineering non-player character. However, some knowledge of a monotheistic deity's plans can prove useful. Allowing knowledge in abstract or obscure ways may serve to heighten tension.

The most well-known method to provide such a limited reveal is prophecy. Because the monotheistic deity sent it, you know it must come true, but you cannot be sure exactly what it means to say must come true. For example, a prophecy that dictates the victory of a red dragon over a white dragon could refer to the literal outcome of a great red dragon over a great white dragon, or it could mean something metaphorical. Perhaps it foretells the victory of some metaphorical red dragon (such as a kingdom whose symbol is a red dragon) over a metaphorical white dragon (such as a diabolical wizard known by the moniker "white dragon" for his attitude and his cruel obsession with cold magic).

Competition for Worship
Tension can also be preserved if the godhead respects the free will of those beneath it. Then, lesser beings have leeway to usurp the faith of mortals. The deity must leave alternatives or else that free will becomes totally meaningless. Challenges to the religion must exist even if challenges to the power behind that religion do not.

Perhaps the best way to maintain conflict in the arena of faith is to keep divine magic from seeming common or cheap. It must be a special reward for exceptional faith. A surprising number of D&D players forget that clerics have to be true believers, basically giving up their lives to their patrons. It's not often an easy thing to partake of a god's power while maintaining your own independent goals, especially really selfish ones, so the evil and the un-devoted will be sorely tempted by "easier" roads to power.

A great arch-devil might patron enemies of a heroic deity-worshiping party, or a godlike fairy queen who neither serves nor opposed a monotheistic deity might patron a PC sorcerer who draws power from his fey blood and likewise stands apart from enemies and servants of the god. The most commonly used "easier road" is fiendish deals, but there are many others. The deity might have rivals for worship that, while minor on the global scale, are nonetheless viable choices for PC or NPC reverence. Sometimes a fey noble can attract his own cult interested in buying a piece of his magical power or celebrating his influence over the natural world. Sometimes alien beings like the Great Old Ones of the Lovecraft mythos or the aberration gods of D&D's Far Realms serve in this capacity instead. Occasionally, even powerful celestials can provide distractions from the deity, if they operate at cross-purposes with him. This may happen if the celestial has significant differences with the deity (such as dramatic differences on the law-chaos spectrum or disagreements about the nature of free will) or the deity is non-good.

Internal Conflict: Heresy and Schisms
Conflict amont the faithful is perhaps the biggest dramatic advantage of the less interventionist god. If the god offers very little feedback to clerics, those clerics may come up with very different ideas of what constitutes proper behavior for a believer. Schisms may develop, one religion splintering into two, three, or even more disagreeing groups. There may even erupt holy wars between competing branches of the same religion, each side believing it to be their duty to wipe out the heresy proposed by their opponents.

The existence of many factions can feed into the same appeal players and DMs find in each of their characters having different patron deities - it provides a shorthand for significant differences in beliefs, ideals, and personal history. Several options remain to provide this kind of variety within a monotheistic milieu. First, as already mentioned, the religion can be splintered by differences of opinion about a god that responds little to settle disputes. Second, and possibly reinforcing the first, a monotheistic deity can have any number of servants with godlike power or at least great significance to different groups of worshipers. The faithful might revere powerful angels such as solars, powerful fiends such as advanced balors or pit fiends, or other mighty spirits that serve as chief lieutenants to the deity and provide direction on issues the deity himself is silent about.

Back to the Beginning
Imagine a world begun with a conflict between the sole goddess and her first child/creation, a vast and powerful elemental, who at first adored his mother but wished for slaves to adore him like he did her. The plot might be driven by this conflict in any number of ways - perhaps the goddess's high priest needs the heroes to crush a dangerous arm of the elemental's cult, or the elemental cult fights on the behalf of innocent mortals persecuted by the intolerant and cruel clergy. In a game at the highest levels, the elemental might even appeal to the heroes to help preserve his influence among mortals against the overwhelming power of his mother's church, revealing dark secrets about her intentions and even hinting that mortal reverence for him is the only reason she has not destroyed the world. A noninterventionist deity might have some very unexpected and unpleasant motives and goals. The aforementioned goddess might wish to simply erase the world and begin anew and is only waiting to see what it takes to win universal adoration before she does it.

A monotheistic deity is often, but not always, responsible for and deeply linked to the way that the world comes to be. Sometimes, beginnings are irrelevant to even the divine aspect of a game or story. It may never come up. However, it can also provide a primeval backdrop for an ultimate conflict, as in the example above. A "prime mover" type of deity might also be simply curious, looking to experiment and learn what happens given certain starting conditions. On the other hand, it might only want to be loved freely and without compulsion by its creations. Or it might seek some other goal dependent upon free will (thus giving leeway for plots to happen without predetermination). The possibilities are numerous.

Becoming Monotheism
Sometimes, a monotheistic deity is a new phenomenon in the world. Perhaps the world once hosted many gods, and over time the rivals have killed each other off, absorbed one another, or have simply been forgotten and faded away. Into the eventual power vacuum has stepped a god that was once less than he is now, with a perspective decidedly different than what arises from inherent omnipotence.

A god new to unrivaled power may more readily use that power and try to reshape the world into his own ideal. Such a shift could even make for a very wonderful place, if the god is very wise and good. However, there is more conflict (and thus far more opportunity for adventures and stories) if a god inexperienced with such power causes problems even if he has the best of intentions. Should the god dominate all aspects of the world, its inhabitants may stop living for themselves and do nothing but try to appease him. They may resent him just because they have no choice in his hegemony and seek out any alternative, giving rise to new and potentially more troublesome religions. Alternately, if the god is not interventionist, he may be nearly indistinguishable from the monotheistic deities described above.

Most gods who begin their existence omnipotent are either quite passive or so balanced and neutral that they might as well be. However, gods with a lower origin often think in more polarized terms. They are also more likely to think on a smaller scale or be downright petty. Whereas a god who starts out on top is usually tolerant of other lesser powers operating in the world to some extent, an up-and-coming young god might not be. He may fear another god taking the same route that he did and so watch or suppress the cults of fading fellow gods and non-divine religions. Or a newly semi-monotheistic god might choose to rule over or leave alone his much-weaker fellows.

Due to his lower origin, this god might not be as strong as other monotheistic gods often are - indeed, he might even be within the reach of epic mortal heroes of sufficient level. Such a threat is most likely when a weaker god is left alone by widespread religious apathy in a setting where gods depend on worship for their power. In a similar setting which retains some fading strands of polytheism, preventing an evil god from being the last god standing might be the goal of a great epic quest. To stop the impending victory, one must revive at least one but preferably several dead or dying religions and thereby restore power to the associated god(s).

Ending Monotheism
Finally, there are many ways for a monotheistic setting to change. Perhaps young, weak gods or epic mortals use trickery and artifacts to usurp the god's position. Perhaps the monotheistic god weakens or dies through lack of worship. Perhaps new gods arrive from other worlds or other realms.

When one god gives way to multiple, more of the tropes of a traditional fantasy setting come into play. Gods feud or form alliances, multiple gods are recognized as having power by all (even if they aren't all given worship), and deities tend to intervene a bit more often. Perhaps two great titans clash for the hearts of the people or simple domination of the heavens, causing cataclysmic destruction with their combat and killing millions as collateral damage.

When a lone great deity is removed, faith will likely not die out altogether. Philosophies without deities at their center may grow to fulfill the desire of the people for something to believe in. Faith in the departed god may even flourish, based on the expectation of the deity's return. If this is a setting where divine truth is shaped by belief, that expectation may even cause the return.

To Go Alone or Not
In the end, the only limitation on the usefulness of a monotheistic god is your own imagination. Such a being can be a subtle but pervasive foundation for the setting, a sort of quiet background assumption, or a rich source of conflict and tension. Should you wish to capture some of the feel of this option but avoid completely dropping the usual fantasy religion tropes, remember that not all religious truths are known to mortals. Indeed, you can even use monotheism and polytheism in the same setting by simply keeping divine influence abstract and subtle. If clerics receive vague or mixed messages from their gods, they are left to draw their own limited conclusions and wonder. And, if you no longer enjoy using monotheism, you can always create more stories in the process by bringing it to a climactic end.