The War Owl Whistle is a quest item in the Dark Dealings in the Blackpits quest, that might make it difficult to find the exact situation on where to use it. It does however give some bonus XP, so it is worth activating. This article will give you a quick answer on where to use it and also the quest around the War Owl Whistle.
Getting the War Owl Whistle
The War Owl Whistle is a quest item that you get in the course of the Dark Dealings in the Blackpits quest. Paladin Thom Hardwin will hand you over the whistle along with a job to investigate on the doings of the White Magisters in the Blackpits. That alone should already be a good hint on where to go. People can however clear out the whole area and still not find the right place to use.
The exact place where you will need the War Owl Whistle
Find the place where Magister Reimond and his accomplice are torturing three black ring members. It is around the harbor in the Blackpits, next to the voidling-fire fight. Two shriekers at the entrance, standing on a post each, will indicate that you are in the right place.
You will have to defeat the shriekers before you can enter and encounter the Magisters and their victims. It should not be much of an issue, but remember that approaching them from the front can turn to trouble real quick.
Once you go past the Shriekers and inside the building, you will see the Magisters and the three Black Ring members. Initiating dialogue will lead to combat so make sure to be prepared. Once you defeat the Magisters, you will have to loot all the corpses and find a specific note that one of the Magisters will drop. Read it and your questlog will update, signalizing that you should inform the Paladin. This is the moment from where you can use the War Owl Whistle to send a message to Paladin Thom.
Alternatively, you can use Spirit Vision and talk to the Black Ring members. The dialogue will also make it possible for you to inform the Paladins via the War Owl Whistle. The only thing that matters is that you find evidence on the doings of the Magisters in the Blackpits.
That being said, it is not absolutely necessary to use the whistle at the earliest moment. You can go and explore the rest of the Black Pits along with various encounters, secrets, riddles, and some extra loot (no spoilers here), Using the War Owl Whistle will net you some extra 18.700 XP, and informing the Paladins and then returning to them will initiate another battle.
Remember that the quest is not completed just by using the War Owl Whistle. There are multiple spots to explore in the Blackpits and you also have to return to the Paladins and talk to them (which will trigger another combat) before the quest can be completed.