Unicorn are a magical manifestation of the life of the Wilderlands. They are a reminder of the potential of life and that all are born as innocents. In general they appear in lands that are in harmony and are universally viewed as a good omen that the realm is blessed.
Only the truly innocent can approach and ride a Unicorn. Otherwise the Unicorn will use its speed and teleport ability to flee. The death of Unicorn produces a supernatural call to all paladins and other divine champions within a 100 miles that great evil is present.
Unicorn, Init +2, AC 2; HD 4+5; HP 19; ATK 3; HTB +4; DMG 1d8 (hooves x2), 1d8 (horn); MV 240’; Save 13; Special: Charge, Magical Immunity, Teleport; Harvest: 5 viz (unicorn horn); CL/XP 5/240; Charge: If a Unicorn moves 120’ or more towards a target and hits with its horn it does double damage. Magical Immunity: Spell casters have to roll a 16 or better on a d20 in order for their spells to effect the unicorn. Teleport: A Unicorn can teleport once a day without error within 360 ft. This ability includes any riders.
Unicorns appear as a white horse with a single ivory horn in the midst of its forehead.
In general, the idea of a lynchpin creature in a fantasy setting doesn't appeal to me. For me the classic example is the unicorn in the movie Legends. A creature so important that the realm is devastated if it is harm or dies. However unicorn are a fantasy staple what I came up with for the Majestic Wilderlands is the idea that they are basically a good omen that things are right. After evil is vanquished, justice is restored then the Unicorns return a visible sign that the good guys have done their job.
This is part of the larger idea in my campaign that magic is in part fueled by emotions and the Unicorns are a sign that the positive emotions are ascendant in a realm.
One idea for a campaign start with a party heavy with paladins and other divine champions is for them to feel the death of a unicorn. An immediate sign that something very bad has happened. Unfortunately the side effect of a Unicorn's death is not very specific as to the details or even the location. However it is enough of an alarm that most of the major bad guys don't go hunting them down unless they have firm control of a region.
To me the Old School Renaissance is not about playing a particular set of rules in a particular way, the dungeon crawl. It is about going back to the roots of our hobby and seeing what we could do differently. What avenues were not explored because of the commercial and personal interests of the game designers of the time.
What are RPGs?
A game where the players play individual characters interacting with a setting with their actions adjudicated by a human referee.
Rules are an aide to help the referee adjudicate actions and to help the players interact with the setting.
Dice are used to inject uncertainty which make a tabletop RPG campaign more interesting than "Let's Pretend".
The only thing a player needs to do to roleplay a character is to act if he or she was really there in the setting in that situation.