Ah, this looks like a good opportunity... (I do this on some alicornified pics, but it's rarer I see a good one to do this for ponies who are already alicorns)
Celestia The Princess of the Day, the Sun Ascendant Greater Deity Alignment: LG Domains: Good, Law, Glory, Sun, Fire, Purification Portfolio: Day, civilization, rulers, peace, work
Celestia is the light of the universe. The pillar of civilization, she is the focus of worship from the dwellers of cities and the inhabitants of farms alike. She can be critical at times, but understands that others can find themselves led astray.
Luna The Princess of the Night, the Full Moon Greater Deity Alignment: CG Domains: Good, Chaos, Dreams, Cold, Community, Repose Portfolio: Night, the night sky, inspiration, freedom, enjoyment
Luna is all that is good about darkness and quiet. A place to relax from the day's work, a wondrous dream, a night spent in fun - all are hers. She provides the counterbalance to the order of her sister Celestia.
Cadence Mi Amore Cadenza, Cadance, The Princess of the Heart Greater Deity Alignment: NG Domains: Mind, Good, Liberation, Community Portfolio: Love, marriage, caretakers, family, music Symbol: As cutie mark
Cadence (or Cadance) is the goddess of affection and love - romantic, platonic, family, and even unrequited. She is seen as the model of a married mare and the guardian of children, as well as representing the process of maturity, and she fosters strong ties between individuals. Her specific focus amongst music is that which is remembered fondly for the memories it evokes. Her consort was originally the mortal brother of the Sorceress, though it is rumored that he has since ascended himself.
Chrysalis The Queen of Darkness, the emotionless one Greater Deity Alignment: NE Domains: Mind, Evil, Hunger, Domination, Madness Portfolio: Apathy, despair, loveless relationships, the Fae Symbol: A crystal heart in a cage
Queen Chrysalis rules the Fae, the most commonly seen of which are the Changelings. She and they feed off strong emotion, drawing it from their targets. While this is necessary for them, they have no concept of how others may feel about it. She has a longstanding feud with Cadence, as the two are effectively opposites. It is interesting to note that neither Chrysalis nor her Fae are "immoral", as they do not comprehend morality - however, they are still generally evil. Some foolhardy and lucky individuals have made deals with her to recieve assistance in return for emotional food. It is vitally important to define the terms beforehand, for obvious reasons.
(I did one of these for each of a fair amount of the recurring cast)