What do you mean with "by the trident"? We already have a Trident in the game which can be used in 3D Sailing Mode to start/end storms. Would you want Zalazar's immortality be turned off when it is used in that way? This would mean that you can disable it during a sea battle during the sea battle. Alternatively, we do also have Blackbeard whose immortality is turned off when attacked with a poisoned weapon like a Borgiablade. The same could be done with Zalazar. This would mean that you can disable it only during a boarding action after the sea battle. Are you adding it to the existing Jack Sparrow storyline? Or is this a new storyline? Or a sidequest available in ALL storylines? Of course if you have it in the existing Sparrow storyline, there is a huge gap of DMC, AWE and OST that isn't accounted for. If that were there, then by the time your story starts, Barbossa would logically already have the ship and sword. But nobody made that yet.