Ok so I'm writing this Oblivion fic and need a convenient poison because my Hok got stabbed and nothing is simple when Martin has to care for someone. (I kinda go on a tangent here so this may will get long.) I wanted to write it all out so this is my excuse to post something for once.
I know nothing about poison so I made one up that would make sense for Martin to be able to go "Oh no, I've seen this before." And it wouldn't seem (too) forced.
So this poison (that isn't always a poison) is called Fool's Ink, or that's it common name among the mortal plane. Fool's Ink is a substance made from the blood of a daedric prince that is diluted with regular ink and something pertaining to the prince themselves. This ink is used to sign the contracts of mortals who bind themselves with a prince.
The contract itself isn't always signing a piece of paper and the ink is never really used as actual ink, unless you're Clavicus Vile. The contract, as well as it's signing, varies from prince to prince.
Martin for example, bound himself to Sanguine by taking a shot of the ink. Sanguine's ink is diluted with brandy so it's not too bad but his blood makes the ink as a whole a potent aphrodisiac. So......yeah... It also makes the veins around entrance site, whether your throat/mouth or in my Hok's case, a few stab wounds, turn a deep, inky, black. (haha, see what I did there?) This is the only commonality with the princes Fool's Ink besides how you make it.
Now Dagon is different than Sanguine. Being the Prince of Destruction and all, he likes to kill things. As well as his followers. So, his blood acts as a poison that weakens the mind and body with a severe fever, loss of appetite, weakness in limbs, really anything kinds sicky. His blood has a unique somewhat magical trait that accentuates any injuries you have. Making putting this on a blade very useful. The reason the poison does not kill you is so your mind is more susceptible to the pull of his vile but cool book, the Mysterium Xarxes. Or just the temptation of the power he could grant you. In my Hok's case, it's the former and not the latter. His ink and blood are diluted with the petals of a dried tulip. According to google they symbolize change and are conveniently harmful to eat.
This is way longer than I intended it to be so if I can think of some good stuff I might make another post with some examples of other princes Fool's Ink. Feel free to expand on this any way you like, just tag me or something so I can see.