What is the price of power?
How far are you willing to go?
How much are you willing to change?
Pacts are contracts between mortals and monsters, an exchange of power or favor to be marked in the core of their being. They are primordial things, perhaps the magic ever woven, something inherent to the soul of both humans and the terrible entity with whom they barter for power. No matter how grand or small, every pact leaves an indelible mark on those who make them, like the threads of a tapestry being unwound and remade. To be
Pacted means becoming different than they were, forever changed from their very core.
Whether that's a good or bad thing is a matter of perspective.
Making a Deal
Well nobody wants to burn in hell
But everybody's got a soul to sell
When I was young my mama gave me some advice
She said, 'boy don't you know everybody's got a price?'
Making a pact can be as easy as finding the right creature and making the right offer. In reality, it is often much more complicated. While every magical creature seems to possess the ability to form such an agreement with mortals, many have no desire to do so. Magi must often convince, cajole, or even coerce spirits and monsters to the negotiation table, where the real test can begin. The process is part bartering, part duel of wits and will, and part magical ritual.
The same quirk of magic that allows pacts to be bound in the first place also prevents them from being coerced. As with many things with magic, intent and will to change reality matters more than spoken words.

by Rawpixel.com (Freepik)
Before any of that can take place, a monster must be found and convinced to talk. In the
Shadow or
Prime, Magi often perform rites of summoning. Others, more elusive entities must be tracked down to the dark corners of the earth - or beyond. For the most powerful pacts, magi must venture into
the Beyond and seek gods or devils in their own homes.
Such journeys are trials in themselves, forming the first step of the pact.
Assuming the magi has found the appropriate entity and survived the encounter, terms are negotiated. Despite depictions in popular media, these are typically straightforward, to the point things rather than long documents shrouded in legalese, designed to trick the unwary and foolish.
More often than not, it's a matter how what and how much, when and how often, duration and conditions. Exactly what the spirit wants depends on its nature, ranging from regular sacrifice to specific acts. Demons always deal in human souls, piecing them up like loose change, while vampires will almost certainly demand a tribute of blood.
Even the most treacherous spirit, once bound by the pact, will abide by its terms. More often than not, it is the human part of the deal that reneges on the arrangement.
Before terms are sealed is another matter entirely. Many monsters have it in their nature to trick, trial, deceive or even devour those who challenge it, and while the terms are always straightforward, the conversation can be anything but.
Once agreement is reached, the entity imparts a piece of itself within the mortal. For most, the transaction is never noticed, but powerful creatures sometimes leave physical marks on their favored mortal. These can take the shape of anything from sprawling and intricate tattoos or elaborate scarification to small physical changes such as changed eyes, hair color, or stranger mutations still. No matter its shape, they brand the magi as touched by a great power.
For some, the change is more subtle. Those who make pacts with vampires might find themselves growing pale, or grow to dislike the touch of the sun, or any other number of small aberrations and quirks. Pacts are never without consequence.
Empowered
Don't mess with me, I've got friends in low places.
Pact can offer many things, ranging from permanent to fleeting. Magi barter for raw magical power to fuel their rituals and spells, strange or blasphemous secrets to develop their art, artifacts, and a dozen things more, from grand to petty.
Power
Magic requires power - sometimes, more than a human can muster in a lifetime. For a price, magi can find this power through the pact.
Spells and rituals cast with such borrowed power are often colored by the essence of its origin. A Vampiric pact-cast spell might leeches life from the world around it, while that of a storm-spirit might raise whipping winds and the rumble of thunder.
Such power is sometimes given as a physical token - a fang, a sealed contract, bottled lightning, or whatever else suitable for the entity in question. Some rites can only be cast with the aid of some specific essence
Knowledge
Even lesser spirits hold secrets mortals can only hope of understanding. To a creature of living fire, the essence of flame is more than just fuel, oxygen and heat, but the universe in microcosm.
To magi who wish to develop their magic, such secrets are more precious than gold. Such arcane knowledge has allowed a thousand kinds of magic to flourish in the shadows, and a thousand more wait to be discovered.
Other desire less esoteric secrets; the location of someone or something, history long since lost, or even details on some other monster, either to slay them or for another pact.
Some think such specific rites or styles of magic were invented by spirits and demons, hoping to lure more magi into dealing with them.
Service
Some magi bind spirits and monsters to act as muscle, assassins, or temporary servants. Such creatures often possess powers hard to match by mortals, magic or not, making them formidable assets.
Walkers often make pacts with minor spirits to act as guides when they travel into unknown parts of the
Shadow, or any part of the Beyond.
Other
Gods and demons can do things that no one else can. Restore lost youth, impart new abilities, remake mortal flesh into new and improved shapes, among a hundred other esoteric things.
Through pacts, magi can become something different than human, both lesser and greater. They trade away pieces of humanity in exchange for power - or healing. Sometimes, there's little choice.
The greatest pact, requiring the greatest price, is the gift of Immortality. Only few spirits possess the necessary power to turn a human into an immortal... Most of the time, with terrible consequences that they really should've seen coming.
JRR Jara
All the details you described made me want to make a pact more than fear it! If I have to give away my soul for being something cooler than a simple human, I take it!
Heck yeah, transhumanism! :D Sometimes, you don't even need to give away your soul!
Creator of Araea, Megacorpolis, and many others.