After meeting with Perikles you have been assigned three new quests. You should begin with the nearest by finding Metiochos at his estate. You will not be able to enter by any door, so you will have to climb the building and jump down into the courtyard. You can then approach him by walking into the only accessible room to the south east.
As you enter the room you will have to kill a number of snakes which surround Metiochos. A few basic light attacks will suffice. When they are dead untie and speak with Metiochos. He will provide you with clues about where you can find the thugs who tried to kill him.
You must now go to the home of the attackers as noted on your map. Once there, you will have to kill three more snakes and interact with their corpses. You can then climb up the house to inspect some correspondence between the thugs and a beneficiary. Finally, go behind the house to inspect a dead body.
When you have investigated all three features, the thugs will arrive and you will have an opportunity to question them. Here you will learn of a snake seller that you must confront, but the way in which you learn of them will depend on your reaction to the answers of the thugs. If you choose not to take a violent approach, you can persuade them to repent for their actions and seek to do good in their local community.
When the thugs are dealt with, make your way to the snake seller to the west. You will find him in a tent outside the city walls. As you approach the tent be cautious of any snakes that may strike you from the grass. when fighting The Poisoner you should avoid his attacks at all costs; he deals significant poison damage over time. Evade incoming attacks and exploit openings in his flow of strikes with a combination of light, heavy and special attacks.
When The Poisoner is dead, return to Metiochos to complete the quest.