1. Power Leveling
Basic - Monokeros is well known and goes by many names, and Unicorn is the most common. At the basic level, the spell summons a single unicorn entity. The summon is not aggressive and will not fight, but it can and will employ healing and curative magics on the behalf of the summoner. Alternately, it will serve as the summoner's mount for a modest amount of time, a minimum of a scene but no longer than a quarter of a day.
Greater Summon - a Monokeros stallion is summoned, and while the summon is more than capable of providing healing abilities, the stallion is more suited to combat and ideally will serve as the war mount of a brave summoner.
Lord of the Morning - A greater Monokeros is summoned, a magnificent beast much larger than a regular monokeros, and has the ability to raise the dead. Like the stallion, a Greater Monokeros can be ridden as a war mount, and has the ability to provide supporting and buffing magic for it's summoner or rider.
Celestial Monokeros - a divine manifestation, the Celestial Monokeros can bring the dead back regardless of their condition, or how long they have been deceased. The divine Monokeros can also grant limited wishes, purify large tracts of land, and otherwise act in a powerful and benevolent manner.
Note - as a mount creature, the monokeros that is summoned need not be ridden by the summoner, another rider may be indicated by the summoner, and the monokeros will treat that person as if they were the summoner for taking instruction. This is fairly common as the majority of Monokeros summoners would be female and less martially inclined. Such summoners could decide to keep a warden or bondsman, a person whom they have a relationship with, and on whom they cast their magics.
2. Stat Boosting
Health - the Monokeros can provide a boost in hit points for the duration of a scene, allowing for a hero to survive horrific wounds.
Stamina/Endurance - for purposes for withstanding poison, disease, and doing good deeds
Charisma - appearance, leadership, an aura of virtue and valor
3. Talent/Skill/Knowledge Boosting
Healing, herbalism, animal handling
4. Hybrid Form
Monokeros is generally a useful, supporting, but lackluster summon. It's untapped potential exists in the Fusion Summon. In this form, the spirit of the Monokeros, noted for virility, power, prowess, courage, and purity, extends this power around either the summoner, or their bondsman.
Standard Hybrid - the subject takes on a mystical aura, and will emanate a luminous halo of a unicorn around themselves. They do not become a human/equine hybrid. In this form, they have an aura of Innocence, which acts in a similar fashion to the Fear spell, but only prevents others from harming the summoner. The hybrid gains bonuses to their magical capability, and their healing magics operate at maximum efficiency.
Greater Hybrid - In this form, the hybrid is wrapped in a suit of armor, and they have the appearance of a white enameled steel unicorn. This is a martial form, and is equipped with magical/holy versions of the subjects normal weapons, or their weapons if said weapons are more powerful than the greater summon's arms. This is the most common manifestation of Monokeros and is part of the mythos of the Knight In Shining Armor.
Lord of the Morning - as per the Greater Hybrid, but the weapons manifested are Holy and in the vicinity of +5 to power. The Lord of the Morning also manifests a Greater Monokeros in full plate barding as a holy mount. The Lord of the Morning has protection from Evil, and evil creatures must make saving throws else flee from the Lord of the Morning.
Celestial Hybrid - this manifestation is exceedingly rare and the spirit of Monokeros fuses with the summoner or bondsman to create the Celestial Guardian. The manifestation appears as a glowing metallic tower like golem. The golem mixes the baroque architecture of a Gothic cathedral and horse barding, with a unicorn motif. In this form, the hybrid emits a constant aura that drives back evil, heals characters of Good alignment, and has an at will Holy beam attack that it can use to devastating effect.
5. Boons and Favors
Monokeros is a pretty straight forward entity in terms of boons and favors. Monokeros can bring the dead back, provide sanctuary or protection for others, and general health, blessings of wellbeing, etc. There are three areas that stand out, however.
An Utterly Loyal and Immaculate Spouse - Monokeros can provide the summoner with a life partner, who will remain utterly and unquestionably faithful, and will allow them to have children, and raise them. There are caveats, the Immaculate Spouse requires the same faith and loyalty, and if betrayed, will vanish. The offspring of a summoner and an immaculate spouse will be technically elemental blooded with an affinity to holy and natural magics.
An actual unicorn - a magical version of the standard summon, to serve as a mount and companion to the summoner. It is common for this to be given to a child of the summoner, as a protector. Said unicorn will remain steadfast so long as the benefactor of the gift remains of Good alignment, and virtuous.
The Arms of the Monokeros - the weapons carried by the hybrid form can be granted as gifts to the worthy. These can be almost any type of weapon, and will be made of ivory, silver, mother of pearl, and other bright celestial materials, and will function as Holy Symbols, and weapons of their normal type with a general +3/+5 vs evil bonus.
Monokeros has no temples, clergy, or shrines. It is an avatar of holy, fae, and natural forces, and its sacred places are in the wilds.
Monokeros will expect their summoner to care for such places, and will offer protection to one if it is needed. To advance from the basic level to the greater summon, the summoner must successfully defend a sacred copse, grove, or grotto from despoiling. To advance from the greater summon to the lordly summon, the summoner must themselves consecrate a sacred place, and ensure its survival. To attain the Celestial summon, the sacred place created must become a natural shrine to Monokeros and all other magical and mystical creatures.
7. Great Summon
The requirements for the Great Summon are difficult. The Great Summon calls forth an Alicorn. Superficially the fusion of a pegasus and a unicorn, or a winged unicorn, the Alicorn has tremendous magical power, capable of granting Greater Wishes, rewriting the past, reversing natural disasters and calamities, or causing them, laying waste to kingdoms as easily as bringing them into new ages of prosperity. To summon the Alicorn, the summoner must have reached the ability to summon the Celestial Monokeros, and must, by that point, have remained a virgin. The need for the Great Summon must also be worthy of summoning the Celestial Alicorn, and it is not to be trifled with. It is much more likely to appear if the summon's gain is for others, and not the benefit of the summoner themselves.