He warned you.
He warned you many, many times.
You thought nothing of it, of course. He had always been cautious, always searching for any possible sign of trouble. How many times had he warned you of some sort of terrifying danger only for the two of you to best whatever foes came your way? How many times had the two of you escaped certain death and returned home safe and sound? How many times had you gone drinking together and laughed about his fears? Too many times to count. He remained ever vigilant, but you grew complacent. You were young and successful and nothing could touch you. You never considered that some things were far worse than death. You never thought that anything would come of this. You hadn't even been expecting to find what the legends and rumors spoke of. Fruit from the Garden of Eden? Something like that could never exist outside of tall tales told by the desperate and the naive.
The bitter irony is that it was to be your last adventure with him. Your respective responsibilities were demanding more and more of your attention and neither of you could afford to waste time on foolhardy journeys. When you told him of your plans, he grew unnerved. The only reason he went on that trip was because of his concern for you. If your safety was his concern, he should have stayed home. Your life is at risk more now than it has ever been.
Your finding it was a stoke of luck. What kind of luck is up for debate. The apple's power was both frightening and amazing. You could hardly process all of the images and ideas passing through your mind as it blinded you. When the light died down and you returned back to reality, you felt enlightened and powerful. He did not. The effects were near instantaneous.
He was blind to the world for several hours before he finally regained his senses. His eyes darted around the chamber as he began to babble incessantly. Nothing you said could comfort him or rouse him from his state. He flinched at your touch and his screams grew louder the closer you came to him. It took a full day's time to convince him to allow you to carry him. You left the apple behind. It was now obvious to you that it was not something that could be controlled by a mortal man. It also occurred to you that being around the apple could hinder his recovery. The further away he was from it the more likely he was to regain his sanity.
He never did. Not entirely anyway. You sought every cure in both Europe and in the East. You talked to anyone who you thought could help from priests to soothsayers. It was no use. He did improve, but he was never the same. What was once a proud and brave warrior was now a paranoid shell of a man barely clinging to his position of power through your help alone.
The apple was your last option. Every other possible option had been sought and had failed. Your fiance begged you not to go. She reassured you that you had done everything that you could and that you could no longer be held responsible after years of hard work. She didn't understand. She couldn't understand the guilt and the pain that haunted your very heart and soul. You thought of a cure constantly. It was all you could think of. You could hardly sleep or eat because of the pain. So you traveled back the temple.
When you arrived, you discovered that you had not been the only one who had listened to the rumors. You swore that you had told no one of what you had found and they welcomed you into their group. Anything to keep the apple a secret. You intended to take the apple and return home as soon as possible, but the Assassin's lapdogs stole it first. You rallied your forces and attempted to take back his last hope for salvation. You failed.
The anguish has nearly killed you. The feeling of being so close to what you sought and having it ripped brutally from your hands has driven you to drastic measures. You have allied yourself with mad men and killers and you have had him transported to the East. He is being kept safe in a well-protected port far away from those that would wish to harm him but close enough that you will be able to get the apple to him faster than the assassins can catch you.
You will pay back that bastard for all that he has done to you. You will succeed in your quest despite the assassin's interference.
Regardless of the devilish people you must ally yourself with, regardless of the lives that will be lost, regardless of what you will have to give up, you will ease his mind and his troubles. Then you will leave the East and its mysteries far, far behind you. You will take him back to the bar where you first met and where you had always met after your travels. You will sit down and drink and laugh and you will do your best to replace his memories of the last five years with joyous thoughts of the future.