(Spoilers continued)
If memory serves, you need two of three 'special items' to get the good ending. Those are the yellow sign, the mask of the pale servant and the psyker crystal. The yellow sign and the mask of the pale servant will also only unlock (I think) if you have a somewhat high insight score. So make sure Trixie spends a lot of time with Parsnip! (the best way to gain insight in the game)