Much like creatures that inhabit the astral plane from which these creatures originate, Eidolons themselves seem to be completely unaffected by the flow of time. However, whomever it possesses does in fact still age, but at a much slower rate, thus making them resistant to magical aging effects, only aging by half instead.

The alignment of an Eidolon can vary extremely, they tend to take on a few traits of who they possess including some of their flaws and ideals and build off of them as they live.

An Eidolon's true form can range from small to large, while they can possess any humanoid planar traveler native to the plane they are currently on. 

An Eidolon has a flying speed of 30ft while in it's astral form and adopts the speed of whomever it possesses.

You have advantage on Saving Throws against being Charmed, you are immune to being possessed, you gain a plus 2 bonus to your wisdom score, and when confronted with magic that can put you to sleep, you can make a wisdom saving throw of the caster's spell save dc to try and reflect the spell back onto them as if you cast it.

While possessing someone, Eidolons don’t need to sleep. Instead, they meditate deeply, remaining semiconscious, for 4 hours a day. After Resting in this way, you gain the same benefit that a human does from 8 hours of sleep.

An Eidolon comes from the mass expanse of nothing that is the astral plane, they are made of nothing and come from nothing. And because of this fact, Eidolons crave being. They are terrified of going back to darkness, back to being unseen shadows, back to being nothing. In order to stay within a physical form they must burn the soul of whomever they inhabit as well as other souls every once in a while of those who traversed an outer plane that is foreign to them, including the astral plane, physically or even projecting there. No one really knows why eidolons crave the souls of extra planar travelers but a prevailing theory is that whenever someone travels into another plane the planar background energies that they are drenched in upon getting there are etched into the very being of the traveler, irreparably changing their soul. Some say that eidolons use this background energy within the traveler's soul in order to ground themselves into the prime material plane.

To consume a soul a humanoid must make a wisdom save contested against the eidolon's wisdom check, on a failed save the humanoid's soul is forfeit, killing the humanoid instantly and making their body smolder and contort as well as burning out their eyes into black husks. On a successful save, the humanoid is immune to this effect for until they traverse a foreign plane once more. Alternatively, the eidolon can slay the humanoid and steal their soul upon its death, however if they already successfully made the wisdom save their soul is protected and the eidolon is unable to consume it. Upon consuming a fresh soul, you must roll 3d10+4 in order to determine how many days until you must feed again. If you eat a soul before a quarter of your time is up, you gain advantage on the new time limit. 

If you go too long without consuming a soul you will start to burn through your inhabited form in several stages.

Eidolons can only exist fully in the prime material plane while possessing a humanoid that has traversed the outer planes or the astral plane via portals or projection. In order for an eidolon to possess someone the humanoid must make a wisdom saving throw contested against an eidolon's wisdom check, the creature has advantage if it is awake or not native to the prime material plane. On a successful save the humanoid can not be possessed by the eidolon until they travel to another plane that is either separate from the plane that they have already visited or if they go into the astral plane (or astral project). On a failed save the eidolon possesses the humanoid and starts burning away their soul as fuel. From then on the eidolon uses the physical stats and features of the humanoid while its class and mental stats stay the same.

If an eidolon is possessing someone for the first time they serve as an imprint for the eidolon's personality, and from then on the eidolon takes on some traits and flaws (at least one of each) of whomever they possess thereafter, perhaps loosing a part of their old selves in the process. As a bonus action the eidolon can make a history check to try and pry into the memory of the humanoid, whether that be specific (such as a secret only this person knew) or general (such as the languages they knew in life) knowledge.

When an Eidolon does not consume a soul within it's time limit it begins to burn its humanoid form away. 

There are four stages to this sickness, beginning the day after the time limit runs out and there is very little one can do to counteract it save obtaining an edible soul and perhaps cooling the mighty fever to stall for time.

The first stage starts with a dc 15 wisdom save that, if they succeed, they must repeat every hour. On a failed save the eidolon starts to become irritable, reckless, and generally anxious or uncomfortable, giving them disadvantage on all persuasion, and perception checks, as well as giving disadvantage to concentrate on spells. Physically, they start feeling ill and their black veins start bulging out, appearing more visibly. They also gain a slight fever and one level of exhaustion.  

The second stage starts an hour after they fail the wisdom save for stage one, make a dc 15 wisdom save that, if they succeed, they must repeat every hour and continue to experience stage one. On a failed save the eidolon starts to talk wildly about whatever is on its mind, a dream they had, a book they read, how they feel right now, what they do in their past time, etc. they might burst out in fits of laughter, hallucinate random things around them, talk to a random stranger as if they were best friends. They will be compelled into madness giving them disadvantage on all wisdom and intelligence checks. Physically their hands and feet begin to turn as black as ash, they will start sweating a black ichor, and they will gain an extreme fever as well as the previous physical conditions, including another level of exhaustion.

The third stage starts an hour after they fail the wisdom save for stage two, make a dc 15 wisdom save that, if they succeed, they must repeat every hour and continue to experience stage two. On a failed save the eidolon becomes extremely happy and optimistic about everything, being in awe at simple things, they stop to smell the roses, all of them. At this point they gain a third level of exhaustion and physically they are now extremely hot, water turning into steam upon contact and 1d6 fire damage to anything you come into contact with, because of this you gain resistance to cold damage. Your irises also appear like there is a multicolored fire right behind your eyes and your limbs and body start withering away and turning even darker in hue.

The fourth and final stage starts an hour after they fail the wisdom save for stage three, make a dc 15 wisdom save that, if they succeed, they must repeat every hour and continue to experience stage three. On a failed save the eidolon gains a fourth level of exhaustion, at this point their bodies start becoming brittle and those black veins are now cracks within their ashy form, from the larger of these cracks, their mouths, eyes, and nose, viscous black oozes out. At this breaking point their eyes are now completely engulfed in multicolored flame and a glowing inferno radiates within the heart of the eidolon, glowing and pulsing beneath the surface of their now blackening bodies.

Due to the fragility of their form, whenever they take bludgeoning, slashing, or piercing damage or attacks with a natural 1 or 20, roll a d20 to see what happens to their limbs:

15-20: Nothing else happens.

10–14: One leg  of the eidolon is disintegrated into ash if it has any legs left, black smoke and ichor shooting from where the limb once was

2–9: One arm of the eidolon is disintegrated into ash if it has any arms left, black smoke and ichor shooting from where the limb once was.

1: The head and body of the eidolon explodes in a fury of flame, smoke and ash. Killing them instantly and summoning a multicolored fireball centered on them.

The eidolon’s speed is halved if it’s missing a leg. If it loses both legs, it falls prone. If it has both arms, it can crawl. With only one arm, it can still crawl, but its speed is halved. With no arms or legs, its speed is 0, and it can’t benefit from bonuses to speed.

The damage to the eidolon's physical form caused by this stage is permanent even if it manages to consume a soul before it dies, the only way to restore limbs severed from this final stage is through a regeneration or wish spell.

If the eidolon is still alive an hour after the fourth stage, make a dc 18 wisdom saving throw, on a success they must repeat every hour and continue to experience stage four. On a failed save the eidolon gains their final level of exhaustion and their bodies explode in a fury of flame, smoke and ash. Killing them instantly and summoning a multicolored fireball centered on them.

Eidolons are unable to lie, however they are not compelled to tell the whole truth either. Due to this they tend to think hard about how they phrase what they say in order to deceive others while still technically telling the truth, because of this they have disadvantage on deception checks. Eidolons can also emit a sort of aura around them giving those within 30ft of them disadvantage on deception checks as well, this aura is a concentration spell up to one minute. Once you use this trait you cannot use it again until the end of a short or long rest.

You can see 30ft into the Astral plane, 60ft of dark vision, and when you are possessing a corporeal form you heighten their senses to their maximum, gaining 120ft of blindsight. As well as being able to sense extra planar travelers within 300ft and can see a colored chord trailing off behind them, the color corresponding to the outer plane(s) that they have been to. You have proficiency in the Perception skill.

An Eidolon mainly has two forms, that of a humanoid planar traveler that it has possessed the body of, and that of it's true, amorphous, astral form.

There are a few warning signs of someone who is possessed by an Eidolon. First their irises turn pitch black with highlights of multicolored light like pools of oil, their blood a viscous black ooze. Eidolons in human form cast no physical shadow but instead their reflections are that of a dark silhouette wherever skin shows through, the eyes of which glow with bright multicolored light.

An Eidolon's true form is little more than a specter, invisible to the naked eye and unable to affect most of the material plane while in such a state, moving around or through things, only bleeding through reality on the edge of the astral plane. Occasionally they can appear as an amorphous silhouette in mirrors, or the long shadow of someone who isn't there, with no apparent source of origin. If an An Eidolon wishes to affect the material plane in some way they must make a dc 16 wisdom check.

If the physical form of an eidolon is destroyed or is otherwise killed, the eidolon must succeed a dc 10 wisdom saving throw or else forget all that they were before and become a blank slate within the astral plane, forever wandering until they stumble upon a new host where they shall begin anew.