For the murder in my plot, I need a fast acting poison (under a day, best case, or over that but doesn't have any largely noticeable symptoms) that doesn't leave much of a trace on a body and is too obscure to identify or doesn't leave much of a chemical trace post-mortem. If anyone knows anything that would be great or if something like that doesn't exist and I should make up a new poison. Thanks.
I would probably go with arsenic. Arsenic is difficult to detect, but because arsenic grows in hair, the amount of arsenic at the scalp and most recently grown hair would have more arsenic than the tip of the victim's hair. If you use arsenic, then then the murderer could cut off the victims' hair so that the arsenic levels can't be compared, although I'm not sure if that goes with your plot.
How fast are you looking for? I have the book "Deadly Doses: a writer's guide to poisons" and there are quite a few listed, along with timeframes and possible reactions along with other notes. I've found this a very useful book to have for writing things like poisonings
So, are you looking for something that works instantly, in minutes, in a couple hours, or takes most of a day?
Arsenic would require a dose of 5-50 mg/kg to be deadly. It generally is found in things like rat poison or such in a white powder form, but can also be inhaled as a dust or gas rather than being ingested. It causes severe gastric distress, vomiting, diarrhea with blood, etc. convulsions and coma come shortly before death and death is usually caused by circulatory failure. Symptoms begin in as little as one half hour and death may occur in as short as a few hours or may take up to 24 hours. Arsenic can be detected in autopsy specimen, even those that are quite old, so it would be possible to go back later and determine it as the cause of death (if you needed that plot twist).
Another good choice would be curare (a plant) or hemlock (another plant). The entire curare plant is toxic and will kill in as little quantity as less than 5 mg/kg. it is almost immediate in its effects and causes paralysis of muscles, starting with the eyelids and face, then moving to the diaphragm and then paralysis of the lungs. It is typical administered as an injection or IV. Labs give diagnosis of enflammed liver. Hemlock plants are just as poisonous (less than 5 mg/kg is lethal), and all parts of the plant are deadly. There are accounts of people accidentally ingesting the leaves in a salad. It has the added benefit of being common in the United States and can be found on the wayside in nature. It too paralyzes the muscles and causes quite a bit of pain in the muscles as you die. Sight is often lost, but the mind remains clear until death, which comes from paralysis of the lungs. The first symptoms start within a half an hour, but death takes several hours (but less than a day).
There's a few ideas, and the book has tons more. I just went for a couple quick ones that might be easy to find without special shopping or licenses or raising any flags. In fact, most of the plants listed would probably work well. Let me know if you have any questions. I just summarized the data from the sections in the book
Depending on when your story is set, you can always inject air into someone's veins (or, better yet, their already established IV line!). It can cause a fatal air embolism which kills fairly immediately, and it won't show up on a tox screen. If it happens to someone with an IV or to someone who injects drugs (or is made to appear to do so), it would probably be ruled as an accident if the ME recognized it as an air embolism.