In the city of Raven Rock you can meet a merchant named Milore Ienth. The woman sells alchemic ingredients and is an alchemist herself, but she's missing one very rare ingredient - the netch jelly. It can be extracted from a freshly hunted netches - the flying armored creatures capable of using simple magic. They can be found in small groups on the island of Solstheim and are the targets of hunters from Skaal Village.
If you don't consider yourself a hunter, you can always rob the citizens of Raven Rock - some of them has netch jelly in their storages (it's randomly divided between houses). A much more interesting solution is to hunt down the creatures and obtain the ingredient yourself (you will need five samples).
Netches can be found mostly on the western and southern edges of Solstheim - a few can be found northwest of Raven Rock and the rest between Tel Mithryn and the mentioned city. The creatures like the proximity of water, so most of them will be near the shore. Be careful, as despite the appearance, these creatures can use powerful magic and might kill you easily!
Bring the netch jelly to Milore Ienth
After obtaining five samples of netch jelly, return with them to Milore Ienth and collect your prize. You will most likely be rewarded with a few strong potions that will help you in your further adventures. For the future, remember that netches are friendly creatures (just like for example mammoths) and hunting them is cruel and completely unjustified.