What's important to the setting?
Rude words are rude only because we decide they are. The word and phrases that a society feels are inappropriate say a lot about the people and culture, so you're going to need to start with a solid understanding of the values and beliefs of the society.
Consider what is commonplace in your setting, what's sacred or profane, what's respected and what's reviled. This is where your profanity comes from.
Look at real-life swearing
Rude words are diverse and multifaceted, but fall into some pretty standard categories and patterns. If we get a working understanding of real-life cussing, it becomes pretty easy to create setting-specific vulgarities. I'll try to avoid using much currently-offensive language in this post, though.
Vulgarity and obscenity
"Vulgar" comes from the Latin for "common," and acquired negative connotations of "ordinary." Mentioning gross bodily functions, sexual acts, and private body parts is vulgar, so we use euphemisms to talk about them. Eventually the euphemism becomes widespread enough to be considered as rude as the word or phrase it was created to avoid saying. A new euphemism is born, and the old loses its potency as it falls out of use.
Thus you'll find that seemingly innocuous phrases will, over time, gain and then lose offensive meanings. A clear modern example is the rise and fall over the last half-century of multiple words and phrases for homosexual acts.
Swearing, blasphemy, and profanity
(It's likely that your "Rust and ruin" example falls into this category.)
Swearing, specifically, started out as an oath, the invocation of something sacred. In some countries, swearing to tell the truth with one hand on a sacred text is still common practice in court. "Oh, my God" and "Jesus!" are also common examples of swears: until pretty recently (historically speaking), they were short but reverent prayers asking for aid and support.
Blasphemy is similar, but disrespectful. For a time in Europe, it was common to swear by the symbolically significant body parts of Christ: "God's blood," "God's wounds." Some particularly daring people swore by other parts of Christ, too. It's also blasphemous to casually invite divine retribution if one is not telling the truth: "May God strike me down," "God blind me!"
Because these phrases were blasphemous they got "minced" to hide their original intent, resulting in phrases we're more familiar with, like "zounds" and "blimey." Even "gosh" is a minced oath for "God."
For many people the use of such phrases is now so common that it's intended as neither invocation nor blasphemy anymore. In a fantasy setting like D&D where religion is even more universal than it was in most parts of the historical world, such degradation is unlikely to happen.
This is too wide a topic to cover, but insults (which often overlap into vulgarity) are very common and sometimes slip into popular usage. Maligning one's parentage, parents, or one's relationship to them is common in most cultures, and often the phrase used to do so becomes a more general form of profanity.
Another form of insult is the pejorative, usually taking the form of a racial or cultural slur. They're often focused on groups that are considered threats: immigrants, ideological or national enemies, and people that need to be "kept in their place." These words are a form of psychological warfare, creating attitudes and patterns of thought in order to retain power. Use them carefully in your games or things can get ugly.
Wishing ill on the guy who made you mad is a long and proud tradition. Diseases, eternal torment, and the ruin or shame of his family or finances, are all common techniques with thousands of years of practice and creativity to draw on. It's hard to find a more comprehensive example than the Curse of Carlisle.
This also undergoes "mincing," as "Damn you to Hell" gives us "darn" and "heck."
Modern swearing is mild and lacks context
A lot of the "bad words" we use today have lost the significance they once had. Part of this is because our society isn't as homogenous in its religious beliefs: profanity, swearing, and blasphemy are a lot more powerful when everybody thinks they're Serious Business. In a god-riddled fantasy setting, such words and phrases are more serious than they ever were in the real world. In a context like that, "mincing" is probably even more common.
On the other hand, references to bodily functions were once pretty tame, but largely thanks to a widespread era of prudery in the 19th and 20th centuries, we have a societal delicateness about those topics.