By the start of Dragon Age: The Veilguard, a contingent of renegade Qunari soldiers known as the Antaam had taken control of the Antivan city of Treviso. Because of how easily Treviso fell, it would appear that the Crows of Antiva now believe a traitor resides behind its city walls.
Suspecting an invasion, the Crows have sent members around the city searching for proof to confirm these suspicions. Two such scouts go by the names of Dareth and Jacobus. The leadership of the Crows haven't heard back from them in quite some time. This is where Rook and The Veilguard enter the scene.
This regional sidequest becomes available only after you recruited Lucanis in Dragon Age: The Veilguard. If you want to know how to start it, just return to Treviso and talk with the leadership of the Crows.
Head to the Drowned District. This should be findable on the south side of Treviso using markers, making "A Slow Poison" your active quest. A waterside warehouse is a starting point. When you overhear an Antaam arguing with Jacobus, you know the journey is going to start.
In order to rescue Jacobus, you're going to have to defeat some Antaam. You'll find some that are vulnerable to necrotic damage, and others that are resistant to electricity; make sure to take Lucanis with you for this side-quest as you receive several bond increases for doing so.
Once the Antaam has been killed, the cage of Jacobus can be unlocked. He requests that you finish his last deal by having him take a glimpse at his cousin Dareth's corpse. Dareth is on the floor of the warehouse, to the left of the cage.
This, in fact, is a pretty straightforward part of the quest. You can skip the following of purple Crow markings and go straight to the task marker in Dragon Age: The Veilguard. Sure, you can come up with another way to reach your destination; Rook will still know what you're doing as you are searching for Dareth's contract.
First, you need to find the contract details in the area before proceeding; these characteristics can be found on the desk in the southwest corner of the room.
Your next two quest markers are to head back to the Crows' lair, and north over a zipline from the casino-if you haven't activated it previously, you may need to do so now. Once across, there's a gap to jump over before you can unlock a door-but this door will only unlock during the quest. You may also reach another zipline leading to the Antaam garrison where the Butcher is giving a rallying speech to his troops.
By the elven gods, the Qamakmaster has been turned into some kind of beastly monster. Even so, his elemental resistances are the same as those of all the other Antaam, and he doesn't have all that much health.
The Qamakmaster is a relatively easy boss to kill. While he is relentless in his pursuit of his prey, Rook, he'll occasionally leave a patch of (necrotic) poison on the ground. This venom stays there for a moment and then disappears. Your best bet against the Qamakmaster-since he shows no mercy and most of his attacks can't be blocked-is to use your companions and special abilities. Other than that, just keep moving around stuff.
When the Qamakmaster has died, Rook will be eager to destroy all of the qamak jars in the room. Instead, go to the northwest corner and unlock the door. It is locked with a key, which you should have received when the Qamakmaster died. The doorway opens on to a large chest.
Then, smash the three jars below, with a ranged weapon if you can. Check out the desk on the west wall - Rook will still be searching the room for evidence. You can leave back to the Crows' lair through the south door after reading the document that's there.
Eventually, Crows' commander has a hot dispute with assumed ruler of the Treviso: the governor of a city. First and second dialog box: say anything you want; third one: choose "I won't abandon my allies." If Lucanis is present that will add some points to your relationship. Here the quest is completed and you get 250 experience point and +250 crows strength.
Now that you know How to Complete A Slow Poison in Dragon Age The Veilguard, follow our steps and you're done.