On this page of our Assassin's Creed Odyssey tutorial you will find a description for "Of Visions and Visionaries" quest. You will start this quest at the same time as the previous one - it's time to visit the famous Delphi Temple.
Consulting a Ghost
Go to the temple. There you will meet Herodotos, a great scholar who is passionate about history. Enter the temple and you will witness a cut-scene and a conversation with Pythia. Regardless of the dialogue, you will be asked to leave the building and have a new conversation with Herodotos. After that, the quest is completed. Reward: +2475 XP.
The Truth Will Out
You must get to the oracle and interrogate her in private. You have to reach the Pythia's house, not far from the Sacred Lands of Apollo.
The place is well guarded. You can try to sneak into a woman's chambers, or eliminate enemies and then quietly go to a meeting. Say goodbye to Pythia and force her to share information. Then go to Herodotos and tell him about everything. Reward: +2475 XP.