To unlock the One Really, Really Bad Day trophy in Assassin's Creed Odyssey, you need to have Update 1.2.1 installed and have completed at least Chapter 5 of the main quest.
The first quest of this arc, "Righting a Wrong", can be found in the White Crystal Hills region of Lokris. The closest viewpoint is Lokris Overwatch. The quest will be highlighted on the map with a blue diamond with an exclamation mark in the middle.
Some of the quests below might be different for you depending on which options you choose. The trophy will unlock regardless of which choices you make, so don't worry if your quests and outcomes aren't exactly the same as I've listed below.Righting a Wrong
Make your way to the quest marker and talk to Megakles. Follow the conversation choosing whichever options you wish. You will be tasked with destroying two pirate ships. You will need to board the Andrestia. If you need to travel to a Ship Dock, the closest one is in Red Lake Bay (Near the Lokris Overwatch viewpoint), Once destroyed, report back to Magakles in the same location that you started the quest. Follow the conversation to complete the quest and unlock three new quests under the "A Town in Need, Indeed" quest.A Horse, of Course
The first quest marker is located very nearby. Talk to Nikaia. She will ask you to retrieve a Stallion from the Salty Bandits Camp which can be found to the east. Locate the horse, you can choose to fight the bandits or use stealth and make a run for it. Return the stallion to Nikaia. Talk to Nikaia once you arrive at the location to complete the quest.Ajax on Fire
The first quest marker can be found in the Great Mount Parnassos region of Phokis. It's close enough to ride without the need to use a viewpoint to fast travel. Talk to Ajax at the location. He'll ask you to escort him to Alponos. Follow him and defeat any enemies you come across along the way. Once you arrive, talk to Ajax. You will have a choice to make at this point. Whichever choice you make will still complete the quest.The Blind Blacksmith
The first quest marker can be found nearby in Alponos. Talk to Kallipides. Carry the three blocks to the first step and talk to Kallipides again. Give Kallipides 10 iron. If you don't have it, you will have to collect some. You will then need to acquire Kallipides' tools from the nearby Pyrrha Salt Pirates Camp in the Hot Gates of Thermopylai region of Malis, which is close enough to ride to without looking for a close viewpoint to fast travel to. Collect the tools by defeating the bandits and return them to Kallipides. After a short cutscene the quest will be completed.
Report to Magakles to complete the A Town in Need, Indeed
quest. I chose to be honest. I'm not sure how much different things will be from this point, if at all.Here Be Pirates
You will have to kill the pirates attacking Alponos. They can be found by the gold coloured triangles on the HUD, and will be highlighted with the same icon above their heads once you are in close proximity. Once they have been defeated, talk to Magakles. You will have a choice to make at this point. Whichever choice you make will still complete the quest. There Can Only Be One
The first quest marker is located nearby. Talk to the other "Eagle Bearer". You will have a choice to make at this point. Whichever choice you make will still complete the quest.
Once you have made your choice, the quest will be completed and the trophy will unlock.