Where can you find the demon who stole the Druid of Agriculture's soul?

Marc, actually the Druid of Agriculture is a long one, first do you already have a boat?? (bring the boat dealers a witch as wife, the boat will cost you 3'000.00)head down to the Highlands and find Velgred's Cottage,talk to Velgred (twice) until he tells you about his stolen glass eye,after you bring him his glass eye, he will open the gate behind his house and this is call the Blackbone Caves(be prepared the monster are very strong) there is th soul, and the frozen body is at the Grenn Rock Temple.

I already have the boat, but where is this man?

I think you can't go to western isle if you have boat that can only bought in 3000. It can't go to deep waters. Buy a boat in Sedona. It cost 10,000. (I think) To go to Sedona, first you have to buy a rune in veldarah.