Do not embark on this quest alone. Find someone you trust; inform them truthfully, and explicitly, of your intentions.
Ask them to read this, if you are able, and make sure that they understand both the risks and the rewards at stake.
Make sure, above all, that they know that one of the two of you is going to die.
It is possible, of course, and even likely, that both of you will die; it is also possible that you'll ask the wrong person. Actually, scratch that -- you probably will ask the wrong person. At least at first. Most people you ask will be horrified. Many will believe you are joking, or crazy. But there's one person out there, somewhere in the great wide world, who will agree to go with you. That person is not even going to ask too many questions, because that person is connected to you and understands your mission as a Seeker and their task to replace you if you are the one to die.
You may not understand how the two of you are connected. You may be surprised to discover that this person's fate is so deeply intertwined with your own. You may be shocked to learn that this is your soulmate.
Imagine how surprised you'll be when the betrayal happens.
Once you two are ready, take to the road and travel through the country until you find an abandoned and ruined church on the left side of a nameless gravel drive. It will be raining when you find it. This trip may take hours, days, weeks or months. Talk to one another while you travel; you want to know every triumph and failure, fear and fantasy of your partner.
Walk inside the church together; you must try to be as close to one another as you can get. At this point, I might even suggest tying yourselves together with a rope -- some Seekers claim to have found it useful. If you get separated at any time after you've entered the church and the trial has begun, you'll never see each other again; the first one of you to give up the search will be doomed to wander the church ruins forever, hungry and alone, shivering in the rain.
If your friend gives up before you do, you'll eventually find yourself back at the car. This could take a very long time.
Walk up to the atrium, get on your knees, close your eyes, and whisper in unison that you wish to see the Holder of Regret.
When you open your eyes, there will be a very old priest in front of you, looking down on you with a grimace of sorrow. A dead twin, ancient and rotted, is conjoined to him. Placing his hands gently on your shoulders, he will very quietly ask, "Are you sure you wish to go on?"
If you have any doubts, or are no longer sure you wish to continue, tell him so and he'll let you go. You may speak to him at great length, if you desire; he makes an excellent counselor. If you tell him a lie, however, the grip on your shoulders will crush you down and you'll meet the demise of all those he deems duplicitous, buried alive and broken inside the walls of the church.
Otherwise, just say "We are sure."
The priest then will turn without saying anything else and lead you to the bottom of the bell tower. There are many bloodstains here, and smears of much fouler things as well. The pavement is cracked from where heavy things have fallen from high above, and rain trickles down into puddles. As you and your friend slowly follow the priest up the spiral staircase, darkness will gather. Soon you will not be able to see even your hand in front of your face, let alone the next step - so be very careful about your footing. There are missing steps here and there, and a fall would mean certain death.
You must help your friend as you climb. Your friend must help you. You will stagger together like drunks in the sagging wet dark, always trusting the other as you crawl one step at a time into the cold infinite. If your trust fails, you will fall; if your friend makes a mistake, you will both die.
You'll climb for hours in damp and shifting darkness, thinking you should have reached the top a long time ago, but the staircase will stretch on for what seems like forever. Keep climbing until your strength runs out. Crawl if necessary, but don't stop. You'll reach the top about the time that the weaker of you has reached the limit of your endurance.
You'll find you have arrived at a circular wooden room, lit only by a single candle on top of a table, beside a glittering knife. Behind the table, a thin man stands hunched and waiting. Get on your knees and ask him "Will they ever forgive?
For an answer, he'll stand up and approach you slowly. There's no telling how or why he chooses, but he'll place his hand over the face of one of you.
Immediately, that person's mind will be filled with brutal depictions of long forgotten but never forgiven sins against life and everything that people hold sacred. Visions of violation and betrayal, in every manner from subtle to sacrificial; every cruelty, every treachery and every lie will be made clear. As the images wash away all hope and humanity from that mind, a burning rage will rise through his or her body, transforming it into a voracious beast that will hiss and tear apart the thin man with vengeful claws and fangs.
That person will speak of what they see. You will never forget what you hear.
This is the only chance that the one of you not touched by the Holder will ever have. You must to run to the table, grab the blade there and shove it into your former friend's heart. Once it is embedded, you must never remove it. As the beast struggles in agonizing pain, set it ablaze with the fire from the candle and sit closely staring for hours as it is slowly reduced to ashes.
Don't take your eyes away from the flaming corpse for a single moment, or you may miss the chance to spot a small black cat rising from the ashes before flickering out of existence - and then all this would have been for naught. Pull the cat from the flames and run down the stairs and away from the church a fast as possible. The church is ablaze, and you don't want to be caught inside as it collapses anew.
The black cat will flicker in and out of existence, and it will often go missing for days at a time. It will never warm to you. Eventually it will disappear for good, never returning. This is because it truly belongs to Legion now, and that is where the cat's true loyalties lie.
In the short amount of time you have the cat, however, you may read the mind of anyone who has some measure of your tissue -- flesh, hair, blood or other fluid -- within their body. Likewise, you may read the mind of another, as long as you have some drop or snip of their tissue within you. Many owners of the black cat begin to collect the nail-clippings, hair, blood and teeth of acquaintances. You will always be able to read the mind of your biological parents and your biological children.
This is often deeply unpleasant.
This ability will only manifest while at least two of the following conditions are met: it is night, it is raining, you are holding a cat, or some part of you is cut with a knife or burnt by a candle. It will never function while in a church.
Every night before you fall asleep, you'll feel the bright yellow eyes of the cat staring at you. Accusing you. And you'll wonder, every time, whether it was your face the Holder touched, after all, and whether it was your voice speaking of monstrosity as you plunged that blade into the heart of your friend.
Credited to Mao