This is the third officer quest you can complete in Master-at-Arms / Padre's questline. To unlock it, you must complete the "Loyalty, Discipline, and Courage" quest. It may also be necessary to advance the main story to a specific point.
The Padre has found the killers. He seeks Edward's assistance to bring justice upon them.
Sail to Cape Bonavista and drop anchor at the marked location.
Hint:
Use the elements protruding from the water and the tree branches to get to the shore. Next, climb the shipwreck located near the waterfall.
You must now kill six of Du Casse's men. Kill the first one when he approaches the edge.
Hide and wait until the soldier standing a bit further away starts moving. When he turns back, move to the tall grass and pull him in when he walks by. Just be careful that the sniper patrolling the rocks on the left side doesn't spot you.
Next, while still keeping an eye on the shooter on the left, kill the brute and hide in the tall grass.
After a moment, the shooter should notice the brute's body and come down to investigate. Kill him while he is doing so.
Climb up to the platform where the shooter on the right is standing and kill him.
Cross to the other side via the posts, sneak up on the second brute from behind, and kill him.
Talk to Padre, who is waiting for you in the village just up ahead. He will inform you about more of Du Casse's men occupying a nearby cave.
There are another six of Du Casse's men inside the cave, and you must kill them all. Start with the one standing on the wooden platform in front of you.
Step onto the leaning tree trunk, from which you can perform a double kill on the two pirates below, but before doing so, hit the patrolling pirate with a sleep dart. There is a good chance that the pirate standing a bit further away won't notice if you do this while the patrolling pirate is near the crates on the right side. After shooting the dart, quickly perform a double assassination.
If the soldier leaning against the tree gains aggro, simply kill him in open combat, but if he doesn't, sneak up on him and assassinate him.
Finally, sneak up to the pirate by the stream and get rid of him as well.
You need to examine three objects. The first one is a crate of guns.
The second is a crate of wine.
And the third is a note - Bishop's Orders - pinned to a tree trunk.
Once you do this, after a short conversation with Padre, this quest will end and the next one will begin automatically - The Other Cheek.