In a book I am writing, I want there to be an mage who magically propels a dagger with a fast-acting, potent poison/venom, which kills one of the main characters. What poison that could be obtained in the mid 17th century that is fast acting should I use?
Poison Dart Frog's "Lipophilic alkaloid toxins"
... is, according to some sources, the group that includes the toxins used by poison dart frogs. They may be able to kill in less than three minutes; you may not be able to achieve any faster.
It's unclear when these frogs were first identified, but Europe touched base in the Americas a hundred years prior to your setting. Knowing that some native peoples employed these toxins in, well, poison darts, it's not unreasonable to believe that some supplies could cross the sea.
Apparently used to tip poisoned arrows in China during your time period, fluid from the Aconitum plant, or Wolf's bane, could be nearly instantaneously deadly if used liberally. The symptoms are not pleasant, and the poison can be used without an arrow and put in food to create the appearance of (accidental) suffocation.
Curare was used by indigenous South American tribes to tips the darts of their blowguns. The active ingredient can take up to 25 minutes to kill a larger animal from respiratory failure, but the tribes added additional ingredients that apparently increased the effectiveness. I read once that the strength of the mixture was determined by the number of seconds it took for a darted monkey to fall out of a tree. 5 second curare was common, but mixtures as strong as 1 second were made.
Don't forget about Tetrodotoxin. This is a naturally occuring poison, found in several sea creatures and was popluar around the far East.
The poison, at a fatal dose, can kill within 15 minutes, causing shortness of breath, paralysis and eventually total respiratory failure. While more of a dose is needed compared to dart frog poison, it has been readily available throught history.
Try strychnine. It was used a couple hundred years ago and can be derived from a plant. It isn't an ingested poison and can cause death very quickly. It is a type of poison known as a neurotoxin, meaning that it attacks the nervous system causing seizures and rapid organ failure. There is no known antidote.
The Bushman Poison Adenium boehmianum from a flower near the Cunene River, Namibia
To quote Wikipedia:
The Bushman Poison is a poisonous succulent endemic to the mostly dry regions of northern Namibia and southern Angola. The San people boil the root sap and latex to prepare arrow poison, which is sufficient for hunting large mammals, as it contains strong cardiotoxic effects. The leaves, borne only for three months a year, are arranged spirally and are clustered near the branch tips. A plant will flower for only a few weeks in winter. The oblong fruit releases many seeds through a longitudinal slit, which due to their lateral tufts, can be dispersed by wind.
The fastest I would know of. Is to remove the jaws of 1,000 long leg spiders. Boil them down to about a baby spoon full. Put on the arrow. There jaws are to small to bite threw human skin. Only thing that saves us. Sea snake would be next, Philippine Cobra after that. You would probably want a paste mix. Adder might also work. Philippine Cobra we do not keep anti venom for. Never been a survivor.
There is nothing more qualified for your novel scenario than the nerve agent, VX. 10 milligrams will kill an adult. It was unknown in the 17th century, but might be used in a story by a Merlin-type character with specialized secret knowledge.