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The situation was a s follows: I was in the shallow water far under the bride of Winterhold, going back to the academy. Suddenly, HP decreasing extremely like I had drunken a
strong poison or a Jarin Root. But I did not consumed anything. No monster to see. I managed to drink a healing potion but the HP drops so fast I even forgot to look in to the magic effects or to make a savegame....drop dead. Anyone has an idea what could happened or experienced the same?

4 Days later: I was able to reproduce it! There is some Poisonous gas (+15 for 3 s) that deals a lot of damage. Strange. No idea from what mod this comes from...