One of the questlines you get in Expeditions: Rome is the Divide and Conquer quest. In this quest, the game requires you to remove Theophilus. One of the optional quests in the questline is to find a poison for Theophilus, and this guide will teach you how to find it.
Before you can start, head to the village of Troas and talk to the NPC called Theodoros. He will be selling spices so ask him what kind of spices he sells. You will need 400 denarii to purchase the poison for Theophilus.
After getting the poison, go back to the Temple of Apollon and head north to talk to the NPC called Zenobia. Keep heading up until you get to the garden beside the water fountain.
Talk to Rhianus and pick the second option that says you want Theophilus as your ally.
After that, say that the Bay leaf herb will make Theophilus kind and generous, this will make Rhianus poison Theophilus, removing him from the throne.
Go back and talk to Zenobia to finish the Divide and Conquer questline.
Well then, assassination complete!