found 2 thieves ring
SPOILER
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slavers dungeon addamasaurus or so
in the loop that collects the main hall (where the door to the slaves is) - with the back hall (where u battle the slaver with the throwing weapon)
TIP to find loop cheat with TFOW in the console
there are 2 mushroom fields
one has coins there u find also a ring
TIP to find 1st ring change resolution to 800x600 at it's a little bit easier
second ring (harder to find)
in the same loop there is also a pond with steps (that ones which are so hard to use) - in this pond there is a skull and another ring nearby
TIP to find it
use night eye, correct gamma, use 800x600
hope it helps