Promise Keeper Achievement/Trophy GuideDuring Act 1, you will make your way to a new area and the first main mission here will be called
Knife in the Dark. You will want to complete this mission and then you will automatically unlock the ability
Death Threat. Now that you have this ability you will want to find a
Worm which one of the green marked enemies on the map which you can interrogate for more information on other captains. Once you select the interrogate option, you can use the
Death Threat option by selecting
which will then allow you to select any captain you want to send it to.
Once the Death Threat has been sent, an exclusive mission will show up for that specific boss. Be warned however, as once you send the threat the captain will become stronger but will also drop better loot once killed. Kill him, and the achievement/trophy is yours.
Here is a video guide for this: