In the Crag fort, you can learn that the captain is very sick and only a Necromancer, who is proficient at healing curses, is capable of helping him. Ciele asks you to go to Karthal and find Kaspar there, who is capable of helping the Pirate King.
To complete this quest, you need to go there (so, you will be able to complete the quest only as late as in the third Act). Once you are inside, go to the Slum and find the Hospice there. Kaspar will join you as your party member.
Return to the Crag with him and he will tell you that, to heal the King, he needs:
- Widowsweep berries(1) - You can find it in the Shadow Woods, to the South-East of Khartal.
- Elven toadstool (2) - You can find them in the Halloth Grove, to the West of the Elemental Forge.
- Sulfur (5) - it can be bought from the alchemist in Seahaven
- Mercury(4) - Can be bought from the shopkeeper in Karthal
- Crystal Spider Heart (3) - 3 level of the Lost City. The door that has, so far, been shut, is open now so, you can get in.
After you enter the Chamber at the City's level 3, which can be opened from the bottom of the map, you will have to fight several undead and banshees, at level 20+, you should have no problems dealing with them. In the next room, there is the Crystal Spider waiting for you. This monstrosity fights at a distance and deals damage for 150-200, which is, additionally, and AoE attack. Therefore, you should not save up on your healing and regeneration potions, to be able to push forward. The most important thing here is to shorten the distance. It does not use its AoE attacks then and attacks individual heroes only, although just as strong.
At some point, it will start to teleport and start being active, out of a sudden. The Spider is very tough, but has few health points. Additionally, it will shut you up in cages two times, which you need to smash to be able to continue fighting.
The first moment is the worst, where you need to strive for short range combat.
After you defeat the spider, you obtain the last item and you can return to the fort.