I've entered the Isle of the Dead to speak to "his" ghost (unnamed for those sensitive to spoilers or whatever) and in order to do so I am given the task of collecting 6 Grave Dust and 5 Black Lotus plants.
I have found 3 Black Lotus plants and 3 Grave Dusts and I have searched the entire island multiple times, I have looked in every crevice I can find.
The first 2 plants are in the center of the area after entering the first gate where the path splits 3 ways, the third I found is next to the stairs to the room that you perform the ritual which requires this potion.
As for the grave dusts, I believe I found 2 in the Arena area and 1 in an area close to the 3rd Black Lotus plant.
Silly me.....I only ever tried to jump up one ledge in this game (If I recall, you couldn't jump ledges in Risen 1 like you could in Gothic) and failed to succeed so I determined they didn't include ledge climbing.
Searching the newly discovered area for the remaining items, hopefully they're all there.