The Aether Graven allies with greater powers, often Primordials from Abeir or beyond, to learn secret methods of investing their items with magical spells and powers. The Aether Graven uses symbols, runes and magic words - etched and engraved onto weapons and armor - to tap into latent magical energies which travel through a primordial medium known as aether. Aether (not to be confused with ether, the substance of the Ethereal plane) is the minimal residual planar energy which permeates the Prime Material plane, allowing light, fire, darkness, cold and other elements to exist and be conjured or controlled. Through semi-permanent alteration of rigid substance (usually metal) the aether is coaxed into a vortex about the Aether Graven Fighter, to be tapped into and wielded as their physical weapons are. 

Engraving Magic

Beginning at 3rd level, you learn to engrave magical symbols, runes and words, using fine minute etchings to enhance your martial tools. You gain proficiency in Jeweler's tools.

You can perform a one-hour ritual to etch a weapon and shield, or a pair of weapons, binding them to you and each other. As a bonus action, whenever you are in physical contact (not necessarily holding) one item, you can summon the other to your open hand. For each weapon, choose a damage type from the following: acid, cold, fire, force, lightning, necrotic, radiant or thunder, and add one point of this damage type to any hit by that weapon. Attempting to bind a new weapon or shield causes one of the other two (player's choice) to become inert and crumble to broken fragments within the next day (DMs discretion).

Once bound, you can engrave your weapons or shield with spells, functioning like a spell-book. A weapon or shield can be used to record one spell-level per pound (two per kg) as listed in the PHB (pp. 145 & 149). Each etching takes one hour per spell level. You can copy spells from a spellbook or scroll to make new etchings, effectively learning new spells. If the spell requires expensive material components (gems, precious metals, ivory, etc.) you must include those in your artwork, affixing the component to your weapon or shield, or inlaying precious metal to the engraving. In order to cast a spell, you must be able to touch the item on which the spell was engraved, and you must have spell slots available to you to cast. See Spellcasting, below for more details.

Spellcasting

When you reach 3rd level, you augment your martial prowess with the ability to cast spells. See PHB chapter 10 for the general rules of spellcasting and chapter 11 for the wizard spell list.

Cantrips. You learn two cantrips of your choice from the wizard spell list. You learn an additional wizard cantrip of your choice at 10th level.

Spell Slots. The Eldritch Knight Spellcasting table shows how many spell slots you have to cast your spells of 1st leveI and higher. To cast one of these spells, you must expend a slot of the spell's level or higher. You regain all expended spell slots when you finish a long rest.
For example, if you know the 1st level spell shield and have a 1st level and a 2nd level spell slot available, you can cast shield using either slot.

Spells Known of 1st-Level and Higher. You know three 1st level wizard spells of your choice, two of which you must choose from the conjuration and transmutation spells on the wizard spell list. You must engrave these spells onto weapons or a shield in order to cast them. Whenever you gain a level in this class, you can learn a new spell (or cantrip) from the conjuration or transmutation schools, which must be of a level you can cast. The spells you learn at 8th, 14th, and 20th level can come from any school of magic. 

Copying Spells. You can copy any spell found in a spellbook or scroll to make an engraving, even if it is not from the wizard spell-list, conjuration or transmutation schools, or a level you cannot cast. A spell takes one hour per level to engrave, doubled if it is not a wizard spell, tripled if it is a level you cannot cast. At the conclusion of the engraving, the original spell must be read as a scroll, destroying the original. Once engraved, if you have spell slots to cast the spell, you can cast it as normal. If you do not have spell slots to cast it, you cast it as a scroll requiring an Arcana check (DC 10 + spell level) and permanently destroying the engraving whether successful or not. If the spell requires expensive material components (gems, precious metals, etc) you must include those in your engraving, setting gems or inlaying metals into the work of art you construct.

Spellcasting Ability. Your spells are dependent upon the skill and accuracy of your engraving, as you trace your finger over magic symbols, runes or words and read them aloud to activate the spell. You use your Dexterity whenever a spell refers to your spellcasting ability. In addition, you use your Dexterity modifier when setting the saving throw DC for a wizard spell you cast and when making an attack roll with one.
Spell save DC = 8 + your proficiency bonus + your Dexterity modifier
Spell attack modifier = your proficiency bonus + your Dexterity modifier

Aether Armor

You have learned to imbue your armor with magic from the aether - nearly invisible aspects of planar energy which persist into the prime material plane. Beginning at 7th level, you can undergo an eight-hour rite to attune your armor to an item you wear (clothing, jewelry, footwear, a belt, something you are unlikely to lose or take off) and then cause the armor to vanish into the aether. You must be wearing the armor for it to be effective against corporeal physical attacks, however it remains effective against incorporeal or magical attacks - including attacks from magical weapons and spells - so long as you are wearing the attuned item. You are not encumbered by, nor is your movement inhibited by armor which is in the aether, thus you have no restrictions to dexterity or stealth because of your armor until it is materialized.

Once attuned, you can summon or dismiss the armor as an action - generally overtop whatever clothing you are wearing, although the armor could appear under something loose like a cloak. If you are not wearing the item to which the armor is attuned, you cannot summon the armor to your person until you place the item back on your body where you wore it at the time of attunement. You cannot summon your armor at all unless you are touching the item, and summoning the armor when the item is not worn correctly causes the armor to appear in a pile at your feet. If the item is destroyed the armor is lost to the aether. 

Close inspection can reveal a faint outline of where your armor was and will be if summoned - a slight shadow, wisp of smoke, puff of mist, layer of ash or dust which clings where the armor would cover - but to the casual observer the armor has disappeared. Passive perception will not reveal the armor without Truesight, and detecting magic will reveal a dweomer upon the attuned item, but does not reveal the armor. An investigation check is required to spot the armor, with a DC equal to your spell save.

Your armor can be used to record one spell-level per four pounds (one per 2kg) rounded down as listed in the PHB (p.145). Each etching takes one hour per spell level. You can copy spells from a spellbook or scroll to make new etchings, effectively learning new spells. If the spell requires expensive material components (gems, precious metals, ivory, etc.) you must include those in your etching, affixing the component to your armor, or inlaying precious metal to the engraving. In order to cast a spell engraved on your armor it must be worn and not in the aether and you must have spell slots available to you to cast. See Spellcasting, above for more details.

Harnessed Aether

At tenth level your contact with the primordial energies of the aether have allowed you to imbue it into your weapons and armor. Each of your bound weapons or shield, as well as your armor, can be used to grant resistance to one of the following energy types: acid, cold, fire, force, lightning, necrotic, radiant or thunder. Imbuing your items this way uses up one available spell-slot on the item, reducing the number of spells you have engraved. Each weapon, shield or armor can be resistant to only one type of energy, and two items cannot grant resistance to the same energy type.

Bound weapons can also  be engraved to add 1d6 damage of one of those types to their strikes, again at the cost of a spell-level being unavailable for engraving. More than one offensive energy type can be engraved on a weapon, each reducing the available spell levels by 1, but each energy type is available only once - two weapons cannot be imbued with the same energy type.

Palpable Aether

At level 15, you have learned to extend and project your perception of the aether, allowing you to cast spells while your armor remains in the aether, and gaining the full benefit of your armor while it remains unencumbering and mostly imperceptible. You have the full armor class bonus from the armor even when it is in the aether. 

Additional Bindings

At level 18 you have learned to imbue a second suit of armor, attuning it to another item you wear. You cannot wear both items at the same time, attempting to do so will break the bindings on both, and cause both suits of armor to appear in a tangled heap at your feet. You may engrave each suit of armor with different spells and energy resistances. Switching between them requires moving one attuned item (jewelry, clothing, accessory) from its' usual place and placing the other attuned item in its' usual place - which can be done as an action.

Additionally, you can bind another weapon or shield, for a total of three weapons or two weapons and a shield, each of which can be engraved with different spells, resistance or damage bonuses, and can sacrifice a second spell-level slot to increase the bonus damage to 2d6 of a chosen energy type. Only one damage type can be increased in this way per weapon. You can wield a maximum of two weapons at a time, or a weapon and a shield, thus you do not gain the benefits of all three items, only those you are holding.