You receieve this quest from Wizardz, who is in the Boardroom of Gods.
He says it will be difficult to find Taran as he hids easily in the night sky.
He says his three henchman named Storm, Drench, and Hail might be easier to deal with.
They can be found near the queen.
To battle Taran successfully, you'll need the Sword of Divine Lightning.
To get the Sword of Divine Lightning, find Credne the Smith in the Ironworkers' Guild in the North.
Once you have the sword, look for Taran.
You might have to search all of Loegria to find him or he may find you first.
You will receive the Kill Taran and his Henchmen quest.