Proposal:
A third Order of Souls voyage type that matches the multi-step complexity and rewards of new voyage types for other trading companies (e.g. Treasure Vaults and Lost Shipments), and aimed at experienced players.
These voyages can be purchased from the Order of Souls representatives at any outpost.
Part One
Finding the Dark Ritual Skull
On starting the voyage, the crew receives a parchment of paper displaying the memories gleaned from a skull by the Order of Souls.
Similar to one of Sudds’ modules in the Legend of the Veil, these memories are displayed as vague images of an island that give a clue to its location.
Travelling to the correct island, the player will be met by a Skeleton Captain encounter with a difference. The Skeleton Captain will be a dark variant, a Dark Captain, and his crew hardened fighters, they will take more hits to vanquish than normal. On defeat the crew will drop Dark Relics whilst the Captain will drop a Dark Ritual Skull.
This skull looks similar to the negative of a Ritual Skull - black with a glowing soulflame-esque teal paint across it. The Dark Ritual Skull can be sold back to the Order of Souls for nice sum of gold (around 2-3K), ending the voyage, or the crew can use it for the next step…
Rough edit to illustrate (original Ritual Skull image was sourced from the Fandom Wiki)
Part Two (Optional)
Performing the Dark Ritual
The second part of the voyage, should the crew wish to endeavour on it, requires finding an “altar” on another island. These altars look like a large stone with a basin on top and strange runes around its base.
To locate this second island will require observing the Dark Ritual Skull as you sail. Unlike other loot that remains static as you sail, the Dark Ritual Skull, as if possessed, will always try to face the altar when it’s not held or confined in a chest. However, like a Wayfinder Compass needle, its movement will be erratic the further away you are from the destination island*.
On reaching the location of the altar and placing the skull within the basin, the final encounter will begin. The skull will ignite within the basin with a Soulflame fire, and pull spectral skeletons from the Sea of the Damned to our world. To make the skeletons physical and fight them, the crew must take the Soulflame from the altar and shine it on the skeletons. The skeletons will each drop a Skull of the Damned and the captains will each drop a Captain Skull of the Damned.
Once the ritual is complete, the Dark Ritual Skull will transform into an Infused Dark Ritual Skull - glowing bright with Soulflame energy it gathered from the defeated skeleton enemies. The crew can now retrieve it once more from the altar.
It's value to the Order of Souls has now increased, netting the crew 5-6K gold should they successfully turn it in (in addition to the other Damned skulls).
*Alternative idea to having the Dark Ritual Skull lead the way to the altar that, I think, would be more straightforward is to have one of the Dark Relics be a Dark Wayfinder Compass.