The Agathion is NOT a classic monster. It is right there opposite the Aboleth at the beginning of the 1e MM2, bit it never made an impression on me, I think probably because it has no illustration.
Reviewing it now with more mature eyes, I see pure role-playing gold. Especially if you are running a psionic campaign, the Agathion is a must-have.
The Agathion has no picture, because it can take any form, even an object. They are basically manifestations of spirit beings who come for the specific purpose of combating some great evil.
I would explain them as stars, who look down upon the earth night after night, observing and seeing, finally putting on an earthly form to take action in some intrigue they have been observing from on high.
The natural form of these living "star creatures" is described as elf-like, with "opalescent, luminous" skin and shining eyes. Your players would only see this form if it was astrally traveling. Otherwise it will appear as follows:
Human form - 60%
Roll randomly to determine appearance.
Clerical spells per day (equal to 7th level cleric with 18 WIS): 1st level - 5, 2nd level - 5, 3th level - 3, 4th level - 2
Creature form - 30%
Attacks and spell powers of the form chosen (such as unicorn, lammasu, shedu, etc.).
Turns undead as 14th level cleric.
Object form - 10%
A ring, talisman, flask, vase, key, etc... Bestows on possessor:
1st level cleric spell ability, detect evil at will, turn undead as 6th level cleric
HD 7+7 (save as 14th level cleric)
attacks: as form, by weapon, or energy touch of 7-14 points
INT: Very to genius (11-18)
Magic resistance: 35% (+1 to hit), immune to level drain, death spells, disintegration, and negative energy attacks
Psionic ability: 177
Psionic powers (14th level):
clairaudience, clairvoyance, object reading, suspend animation, aura alteration, dimension walk, mind bar, telempathic projection
Telepathy (any language)