The campaign world is a caricature of Earth in the late Medieval period (14h-15h century). Some countries are a bit depressed in how history progressed, while others are slightly more advanced as suits the purpose of the narrative.
The pilgrimage site: Long ago a star fell from the skies, and crashed in a fiery ball in the heart of a dark forest. Some took the starfall to be a sign and made pilgrimages to the site, but the way was difficult and fraught with danger. The Church denied any significance to the area, and actively opposed the idea of people taking pilgrimage to the site. Despite the Church's disapproval and many people never returning from their pilgrimages; others returned with tales of wonder, or experiences beyond description. Word of mouth spread, and the site continued to be popular. As a mitigation, A group of knights, tired and worn from their participation in the crusades were given the sinecure of guiding pilgrimages to and from the site, and a keep was erected for their use, as well as a waypoint for the pilgrims.
The players: The players take the roll of 1st level assistants to the knights of the keep. They are the squires and attendants of the knights or the keep. Either base born and given over to service of the church, or nobles sent to glean experience from their battle tested relatives.
The Keep: The keep consists of a stocky stone tower with a small courtyard surrounded by an earthen-work and wood wall. Outside the wall is a small training yard. The knights provide an escort to and from the pilgrimage site for any who desire it.
Sir Klaus von Ecke. Klaus is the official representative of the church. He is charged with the collection of tithes from the pilgrims as well as filing all official reports with the church. He is ultimately responsible for all actions at the keep. He believes fiercely in the code of chivalry and will drill those principles in constantly, applying them to all aspects of life to the squires.
Sister Sabine - Sister Sabine is a nun turned warrior. She keeps all the records of the dealings and events of the keep. She keeps an accurate and truthful record of all dealings. She also has a locked chest that contains all of the magical items that belong to the knights.
Sir Dieter the Short. Dieter in addition to being a warrior was trained as a healer. He has a good knowledge of herbs, can stitch up wounds and even perform minor surgery.
Sven the Blacksmith. Sven is a Norwegian and speaks fluently but with a thick accent. He performs all the maintenance on the keep in addition to tending to the arms and armor of the knights.
Bertha the Innkeeper. Sven's wife. She makes a point of always having a stew going for and pilgrims who show up in the dead of night, or return from the forest at odd hours. She also sells trinkets and pilgrim badges to passing travelers. She is also a witch of no small skill.
All of the staff are aware of the true magical nature of the world and especially the woods. They will keep this from the PCs until they experience it themselves. They keep two records, the official one that is passed to the church, which is sterile and mundane, and a second that is kept for themselves that records any strange occurrences experienced by them or the pilgrims. While on crusade the knights encountered things from beyond this reality, fought and won. By happenstance they had encountered Bertha who traveled to the area through the cave to seek out the meteor for herself. They decided to protect the area, so they could further explore the mysteries, and the best way to do so was to protect it from the church by protecting it for the church. They officially deny the existence of any pilgrim who disappears on their own. They will adamantly declare that the never arrived at the keep if anyone comes looking for them. If the pilgrim had been traveling as part of a group, the knights will mount an expedition into the forest to try to recover the lost soul.
Additional duties assigned to PCs:
The Dark Forrest:
The forest is large and contains many dangers. A path leads from the keep to the site of the meteor's crater. Sticking to the path results in a relatively safe journey. An encounter with a wild animal is a very real possibility, but stranger still are the shadows and movement are caught out of the corner of eyes, the whispering voices deeper in the woods. For these are the fairy folk and many tell stories of secrets revealed in their whispers and tricks of the eyes.
The space rock emanates a strange radiation that can distort time and space. It fluctuates randomly. Sometimes making a pilgrim's visit of hours only take minutes, or sometimes days. Other times it will cause disorientation and a pilgrim will wander off into the forest rather than returning on the path.
The dark cave.
There is a cave in the forest that is never occupied by animal. For it is a gateway to the underneath, and the night market.
Occupants of the dark forest:
The green men. An wild eyed fraternal order of magic users that live within the woods. They appear to be mad hermits when encountered. Dirty and naked, with writing all over their skin. They inscribe their spells onto their very skin. They may cast any tattooed spells without necessitating memorizing first. Lost pilgrims may be accepted into their order.
The fey folk. A group of elves live in the forest and toy with the pilgrims that make the journey through the forest.
The macro view:
The Catholic Church: The Church knows and actively suppresses the fact magic and fairy creatures exist. While information about the truth is selectively distributed to trusted agents, the church as a whole actively seeks to stamp out black magic and any trace of the supernatural. The inquisition and witch hunters are very much actively seeking those that commune with beings from beyond. They seek to eliminate these things both out of fear of the unknown, as well as a genuine desire to protect the status quo of reality.
The Empire (Germany):
The setting is the late Medieval period of Earth (14~15th Century). But not the Medieval period as we look back upon it from our modern conveniences. The Medieval period as the people then believed it to be. Lords and Ladies and Castles and Knights are still a big deal, the black death has depopulated a vast number of the population and fear still rules the day. Is the plague actually over? The Lords are concerned with war and the common man is concerned with scraping enough food from the ground to make it through the winter. The nights are dark and every townsperson has heard a story of the town over that had a witch problem, or a troll under the bridge, but that is that towns problem, not theirs. Stepmother's are pretty much always wicked here.
The night market:
The night market is a place between worlds. Like Diagon alley mixed with a fish market, vendors hawk their wares from across the world. The strange and wondrous can be purchased here as well as sold. The night market connects to many other locations throughout the Earth as well as links to the Dark.
The Dark: A place of perpetual night with no stars or moon. Ruled by Monster Gods.
Black Lotus: An opium like narcotic that has the chance to transport the user to another realm, both in mind and body.
Fighter - No changes
Thief - No changes
Cleric - use magic user
Magic User - d6 hit dice. Can learn all spell types (divine/arcane/other), no starting spells.
Races: Human only
Magic. A character of any level can cast any level of spell provided they have learned it. If the spell is above the character's class ability to cast (either due to spell level or # of spells per day) then they must make a save vs. magic. On a successful roll the spell is cast as normal. On a failure, the character must roll of the following table. For each spell cast in a day this way add a cumulative +1 to the target number.
Saving throw fails by:
1-2 spell fails
3-4 Take non lethal hp damage = to spell level; If spell level is greater than character level, take lethal damage = spell level
5-6 spell is changed to summon, HD of summoned creature equals the level of the spell attempted. The creature is hostile
7-8 take non lethal damage = 2x spell level; If spell level is greater than character level, take lethal damage = spell level
9-10+ roll dx
1 a random spell takes effect (1d10 for level of spell, on a 10 double the level of the spell caster 13-14 for spell effects and roll again)
2 The spell caster permanently looses memory of the spell
3 The spell caster gains insight into the realm of magic and gains a random spell, which is cast at this time instead
4 The spell caster mutates gaining one inhuman effect
5 The spell caster erupts in a flaming ball of magic, extending 30'. Each target within the sphere
So why be a magic user? Magic users have sharp enough minds and strong enough wills to master some of the subtleties of magic. They may memorize spells as cast the appropriate number of spells per day as their class ability without making a saving throw.