a while back I used to play this game with the same mod. I used to use this item which slowed down everything except the player, it worked liked the black ore from gothic 2 NOTR.

The problem is, is that I don't remember what item it was... I tried searching the item code page in the dark mysteries website but I couldn't find any... I did find the black ore but it just makes you float.

PS: here is the link to that item page
http://pl.poziomkaz.pl/articles/324