Swearing, cursing, whatever you call it, colorful language can make a fictional setting either come alive or sound really dumb. Using ordinary 20th and 21st century "bad words" is jarring and breaks immersion, so most writers and authors dealing with alternate settings utilize made-up vocabulary for the purpose. It's also helpful for television and game media to bypass the self-appointed keepers of audio morality.
The difficulty is that many creators don't have a strong grasp of what makes good foul language or why we use it, so we end up with characters who overuse a single word, like "Shards" in McCaffery's Dragonriders series of books. Swearing needs to be tied to the core values of a fictional society to feel like it fits. There are several purposes and types of good cursing.
Is an implied exclamation point or all-caps to your speech insufficient? Scatter in a creative variety of imprecations and obscenities. These are most commonly delivered with strength, so choose words with hard consonants and hissing sibilants. Shock value can also be important here, though a habitual user of this type of swearing has become completely inured to their own speech.
To imprecate is to invoke ill fortune on another. "Gods damn you" and "demons take you" are weak examples, iterating slightly on earthly maledictions.
Swearing is frequently used to accuse the subject of ill-doing or unseemly habits or to declare them anathema. This includes denunciation of ancestry, declaring parentage unsuitable or somehow unclean.
Forbidden topics, such as sex or excrement, are common to the rough language of every known earthly culture. Excrement in particular makes sense from a primitive public health standpoint.
Oaths are some of the more interesting sources of colorful speech, sometimes escaping the label of cursing, sometimes not. Declaring "Amen" or "So say we all" during or in response to oratory is a good example of an oath.
Irreverence toward religious figures, artifacts, and beliefs is also a very common source of colorful language, and frequently the target of much greater disapproval by community leaders who may feel that they owe their status to religious authority of some sort.
Many of the other types may be used in an outright slanderous way. The target of your ire need not be "thrice damned" to be declared so.
Mix & Match
These simple types of swearing are the most fun when combined creatively. Oaths and blasphemy go very well together in "Od's Blood!" Verbal punctuation is frequently combined with taboo. "I want every swinging Richard on the front lines by dawn!" Put together three, four, or even five types for some really interesting verbage.
The point is to think about each category with respect to the fictional culture in question. Start with a few simple words for each, even if they sound a little dumb by themselves. Then use these building blocks to construct more elaborate combinations.
For good examples, I refer you to Robert Jordan's Wheel of Time series of books. Jordan repurposes perfectly ordinary words into combinations and contexts that make the speakers' meaning clear and faces them with genteel disapproval to drive it home. His characters swear by the Light, the Dark One, Blood and Bloody Ashes, and more in a reasonable array strong language.