Café de Nuit

As the topic says, what can a fiend do in its reality wrinkle? How far can it alter what lies within it? Very general question, I know, but I have nothing to really go on I don't think.

As far as I can remember (I don't have my books with me at the moment), your average fiend gains no powers from simply having a reality wrinkle, although it can cross closed domain borders and prevent darklords from using some of their powers within the wrinkle. However, fiends can perform power rituals to gain land-based powers (and halve their reality wrinkle, and make it more difficult to leave Ravenloft. This is all described in chapter 5 of the RLCS/RLPHB). Only a few land-based powers were included in the RLCS, the rest can be found in the appendix of the Kargatane netbook Children of the Night: Demons.

Much of the effect of the wrinkle is atmosphere, it is used by the DM to establish a feeling of wrongness and corruption. At most a fiend could use it as a distraction or disorientating effect.
Given the wide variety and powers of a fiend I'm sure you can attribute whatever abilities and powers fit the theme and mood, reality wrinkles are permission to bend the rules just a teeny-tiny bit.