Monster Manual II said:Agathia take a material form as desired. One encountered will typically be in human form, in the form of a beneficent creature (such as a lammasu, shedu, or unicorn), or in non-corporeal form and contained in a ring, talisman, or possibly a small vase or flask, etc.
Human form is 60% likely. In this form, the agathion will use weapons typical of a cleric and have spell powers and abilities equal to that of a 7th-level cleric. As the agathion's wisdom equals 18, bonus spells are applicable. The appearance of the agathion can be young or old, male or female, handsome or plain, rustic or urbane.
In creature form - but 30%probable - the agathion will have whatever attack forms that creature possesses and the creature's spell abilities, if any, and the powers to turn undead of a 14th-level cleric.
Only 10% of the time will an agathion be encountered in non-corporeal form and contained within some object. When thus present, the agathion is able to bestow its 1st-level cleric spell ability on the possessor of the item, automatically detect evil, and allow the individual to affect undead as if he or she were a 6th level cleric.
Monstrous Compendium 8 said:Agathinon will never attack in their natural form. At the slightest sign of danger, they will assume another form. They gain great benefit from this because they will possess all attack forms and spell-like powers of the form they assume, though they retain their original hit points, THAC0, intelligence, etc. For example, if an agathinon assumes the form of an old bronze dragon, he will have two claw attacks (for 1-8 each), a bite (for 4-14), snatch, kick, wing buffet, and tail slap. It will also have the powerful breath weapon and spell-like powers of the dragon. It will not, however have 18 hit dice, but rather its own 8 hit dice.
On rare occasions, an agthion will assume the form of an inanimate object of magical nature, usually to be carried by some other being. This might be a magical lamp, a magical sword, necklace, or even something as unpretentious as a vase or glass vial, as the agathinon chooses. When in this form, the agathinon confers all powers of the object to its possessor plus several other benefits: the ability to cast 1st level priest spells from any sphere at will, and the ability for the individual to turn undead as if he or she were a 7th-level priest. Agathinon will never confer abilities to evil individuals. In fact, any evil person touching the item will receive 1-12 hit points of damage with no save allowed. Neutrals may receive benefits from the item only if their current mission or actions serve the needs of the agathinon.
Planescape Monstrous Compendium I said:Agathinon never attack in their natural form. At any sign of danger, they assume another form. They have all attack forms and spell- like powers of the form they assume, though they retain their original hit points, THAC0, Intelligence, and other statistics. For example, an agathinon who assumes the form of an old bronze dragon has two claw attacks (1d8 each), a bite (for 2d6+2), snatch, wing buffet, and tail slap. He also has the powerful breath weapon and spell-like powers of the dragon. He does not, however, have 18 Hit Dice, but rather his own 8 Hit Dice.
Rarely, an agathinon assumes the form of a magical inanimate object, usually carried by some other being: a magical lamp, sword, necklace, vase, or glass a vial. In this form the
agathinon confers all powers of the object on its possessor, plus several other benefits: the ability to cast 1st-level priest spells from any sphere at will, and to turn undead as a 7th-level priest. Agathinon never confer abilities on evil individuals. In fact, any evil person touching the item takes 1d12 points of damage (no save allowed). Neutrals may receive benefits from the item only if their current mission or actions serve the needs of the agathinon.
BOZ said:firstly, time to hammer this part out:
I had originally given them these for the form changing ability:
Shapechange (Su): An agathion can shapechange, as the spell, at will. The agathion usually uses this power to assume the shape of a creature, though it can even become an object. Unlike the spell, this ability also allows the agathion to assume Spell-like abilities and Supernatural powers of the form, such as breath weapons and gaze attacks.
An agathion that changes into an object will form a special bond with one character of good alignment (or a neutral character, if her current actions suit the agathion's mission). This bond allows the agathion to transfer some of its powers, as well as any powers the item has, to the character. Any evil being touching the items suffers 1d12 points of damage (no save).
Bestow Spells (Su): An agathion in the form of an object in the possession of a good creature, may bestow the ability to cast divine spells as a 1st-level cleric to the creature. The agathion in the form on an object may also bestow upon the creature the ability to turn undead as a 6th-level cleric.
It does need to have a shape changing power that allows it to gain Su and Sp abilities. Polymorph any object (self only) also springs to mind. There seem to be specific conditions, also, which apply to various forms chosen.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.