Here's the Dark Ages D&D house rules I'm thinking of:
Literacy: because of the oral tradition and rarity of books and writing, all characters are illiterate unless they take a certain proficiency: Theology or Language (Latin). Other proficiencies might allow the character to read ancient writings or write in the vulgate, in the DM's discretion. Additionally, all clerics can read the vulgate whether or not they have the literacy proficiencies, but not all clerics can write in Latin.
Spell preparation: Clerics prepare and cast spells as normal. For wizards and mages, they can cast a number of spells per level as determined by their class level plus their Intelligence bonus. So, a wizard that can cast 2 level 1 spells, 2 level 2 spells and 1 level 3 spell with a +1 Int bonus can cast 3 level 1 and level 2 spells and 2 level 3 spells (i.e. cast daily = spells + int bonus). Every day the wizard needs a few hours to gather spell components, practice reciting magic words, and so on to restore the spells. Additionally, the wizard must sleep for at least 6 hours to recover the magical energies he expends casting spells (as the wizard increases in power, so does his ability to channel magical energies on the same amount of sleep). A wizard can attempt to cast 1 more than the usual number of spells per day with a successful Loremastery check, but this ages him 4+spell level years (the aging rules apply to wizards if no one else).
However, wizards are limited by what spells they have in their repertoire (usually in the form of a spellbook). A starting wizard begins with Read Languages and two spells given by the wizard's master or trainer (roll or pick at DM's choice). Additional spells must be physically added to the repetoire by writing them down, or carving them on a wyrdstone or staff, after deciphering them. Magic spells are jealously guarded and kept secret, so even after a spell is found it must be comprehended and "translated" into a personalized form. This requires a week of research for each spell and a successful Loremastery check. The spell is lost if the check is failed, and another source must be found to acquire the spell. No check is required if the wizard is taught the spell for the week by a master of it, like a senior wizard.
Wizards in the Dark Ages: Arcane spellcasters come in two types, wizards and the Hermetic/Neo-Platonic/Egyptian mages of the south. Wizards, which includes druids, generally derive their power from the spirits and places of power of the natural world, worship the old gods like Odin or Kereneos, and are often illiterate, as compared to the educated Hermetic wizards of the Mediterranean. Sometimes though wizards have their own scripts or secret lores from their own homelands, like Ogham. Both keep their magical knowledge secret, and it requires sacrifice and a mystical connection to cast spells. Both are often relied upon by pagan rulers or rich pagans for advice, fortune telling, and knowledge of monsters, fairies and gods. Rather than always enchanting items themselves, wizards often seek out spirits or dwarves who do the work for them, with the wizard serving as a middle man. Additionally, wizards often keep their spell knowledge stored on different items than books like seeing stones, carved into their staves, or in secret places like the hollow of a tree. A wizard can hold up to 5 spells on a staff or through the combinations found on a small bag of stones, or 3 on a wand or stick. Standing stones or trees can basically hold 15-20 spells.
HP: all characters start with maximum first level hp (e.g. 8 hp + Con bonus for fighters). All the thief-based classes have d6 for hit die rather than d4.
Ability score damage: In addition to the effects of the Mortal Wounds table, a character brought to 0 hp through a critical hit must roll on the body parts die to determine ability score damage taken. Hands or legs reduce Dex by 1, Belly reduces Con by 1, Head reduces Int, Chest and arms Str. Additionally, any permanent affect on a character's sanity or faith reduces Wisdom.
Cleave: anytime a character kills an enemy in hand to hand combat, he can roll again to attack any foe within a step's distance (approximately 5 feet). The number of cleaves is not limited by the character's level.
Shields: A regular round shield can guard against up to 2 enemies' melee attacks a round. A kite shield can protect against any number. A regular shield can be strapped to the back to add +1 AC to any attack coming from behind the character (no regular AC bonus).
Weapons: Unless a masterwork or magical weapon, all weapons except for swords break on a natural roll of 1. When rolling a critical, the weapons have different effects:
Axes - Choose to destroy an item on the target, like a shield, piece of armor (armor perm. reduced by 1 pt AC), or a held item. the victim must also save or be stunned from impact for 1 round. This does not apply to enemy weapons.
Hammers and maces - the target is stunned and, if failing a save, knocked prone, for their next round.
Spears - the victim is impaled with the spear. If still fighting the same victim next round, the warrior can attack the victim by ripping the spear back out with their attack, automatically hitting and rolling damage.
Swords and daggers - The attacker can disarm or break the victim's weapon (not natural weapons though) or impale the victim with the weapon as a spear, attacker's choice.
Bows - criticals with a bow immediately allow another attack to the same or different target in the same trajectory. Generally, bows can be fired into melee at a -2 penalty, Precision shot proficiency removes the penalty. A miss when firing into melee has a 50% chance to hit a friendly attacker, determined at DM discretion.
Healing: Usual healing is 1 hp per night with good rest. Drinking a draught off a meadhorn or wineskin during a short rest will heal 1d4 hp, usuable once a day. After one week good rest, all hp are restored no matter what. Faeries and spirits, in addition to clerics, offer healing, but they exact a price, generally a geas.
Silver standard: the silver penny or denarius is equivalent to the D&D gold piece. An actual gold piece or solidius is worth 100 silver pennies (100 gp). XP progession is normal, with acquiring 1 sp equal to 1 XP. Spending on feasting or gift-giving is worth an addition XP per sp spent. Magic items, although usually not sellable, are worth their price/xp when found (this includes healing poultices and so on).
Communication: everyone is presumed to speak a germanic pidgin that allows them to communicate, although not understand complicated matters in a non-native tongue (i.e. a Gaelic speaker trying to understand a viking king's speech).
forgot a couple
Fighting from Horseback: When charging with a spear (there are no lances), the warrior uses the horse's strength rather than his own. This amounts to a +4 damage bonus, or a +5 from a heavy warhorse. Additionally, mounted warriors enjoy a +1 AC against attackers from foot.
Reaction & Morale Rolls Use a 1d12 rather than 2d6. The bell curve on 2d6 makes reaction and morale checks too boring. Having bonuses from charisma or discipline should actually matter.
