Seal of passage is needed to access the island.
Go to the most western dock at Rellekka, speak to Lokar, then speak to the Captain.
Go to the southern ones on Lunar Isle, as they do not use magic.
They can be ranged over the edge of the bridges. If you range directly from the bridges, they run away. You have to stand on the end squares of bridge, otherwise they run away too.
Usually just drop Suqah hide, big bones, and Suqah tooth, and occasionally guam / marrentil.
Also, here's a weird glitch I get when talking to NPCs with slayer mask on. Reminds me of a spider...