I love that seed thing 
Honestly, I think thw two entries (Gravedirt and this one) are so different they deserve a place of their own, both of them. Think in adventure ideas: If you'd use each item as the basic idea for an adventure, two entirely different adventures would come out...
The Eternal Heartstone
It's known that Night Hags carry so-called Heartstones with them, which not only grants them (exclusively) the ability to become ethereal, but which can also heal any disease and provide a +2 resistance bonus on all saving throws. Those latter powers can usually be used up to ten times (no matter the kind of use) if it falls in the hands of someone else than a Night Hag.
There is one known exception: The Eternal Heartstone. Thousands of years ago, a brave female paladin fought against a Night Hag, and killed her. She took the Heartstone and brought it to the Plane of Radiance, where it was purified for a couple thousand years, and now it bestows perfect immunity to diseases and an infinite +2 resistance bonus to saving throws to its wearer.
An adventuring group found the Eternal Heartstone about two hundred years ago, and discovered its history through the use of minor divinations. It has, since then, changed owner a couple of times, and is said to be currently "on market" somewhere.
The Dark that noone knows is, the paladin never killed the Night Hag. The half-elven warrior knew she was too weak at that point to win a direct battle against a Night Hag, so she used a trick: She forced the Hag's essence into her own Heartstone, capturing her and putting a curse upon her - the Night Hag should only be able to leave her Heartstone at the dark of night.
Then, the paladin travelled to the plane of radiance, and left the Heartstone in eternal light.
It worked for a couple thousand years. But now that the Heartstone has brought back to the Outer Planes, the Night Hag is once more able to leave her Heartstone. Every night, if unobserved, the old crone slips out of the stone in ethereal form.
She seems to be quite happy with her current situation, enjoying to secretly give nightmares to the person owning her heartstone, or even turning material again, casting a sleep spell on her victim, and biting it for a Constitution drain - or both.
Being linked to the Heartstone, even killing the Night Hag doesn't end the trouble, for her soul simply slips back into the Heartstone, and the creature reform in 2d10+5 days. There is only one way to defeat her for sure: Her name must be spoken out loud in front of the Heartstone to break the curse, and then she must be killed.
The only problem is, the paladin made the Night Hag herself forget her name through divine magic, and it would take a good deal of research to find out her name. Even then, she'd still be quite an opponent in direct battle, not to forget that, if she carries her Heartstone, she can escape by turning ethereal...
Hey, I really like that one. It's a useful magic item with a great backstory, and a built in one shot quest whenever the DM needs one. Just one thing:
I think you mean left it in eternal light. Just a minor typo
Ok, it occurrs to me that I've started posting way too often, but I'm bored this weekend, so oh well. After the last item I posted, I realized that I really like the aspect of Hags as sort of a corrupted druid, twisting nature into forms that they believe are better or more useful. I also realized that most of the items here, once the PCs figure out what's going on, would never be knowingly used. Here's something that's very tempting, even if it is dangerous.
Razorvine Panacea
This item is a three foot length of a dark gray variety of razorvine that only grows in the woods of the second layer of the Wastes. If wrapped tightly around the forearm of a being suffering from any disease, its thorns lacerate the flesh deeply and cause the disease bearing blood to ooze forth. This allows the infected being to make a Fortitude save against the disease every minute instead of every day, suffering the usual ability damage for failed saves. The disease is considered cured if 2 consecutive saves are passed. While this is a quick and cheap method of curing oneself, it is also dangerous since the disease can quickly ravage a body that fails a few saves. The process is painful and causes 1d10 damage for every minute that the razorvine remains wrapped around the arm. One length of Razorvine Panacea can be used as many times as desired, but tends to dry out and become brittle and useless after 2d6+6 days. This item may not proove effective against certain magical diseases, or any disease created specifically by a Night Hag.
Lying Nettles
Among the third gloom's various flora and fauna is a unique plant that has discovered an untapped resource from which to attain its nourishment: Lies.
Lying nettle appears as thorny brown brambles laced with razor-sharp barbs that can cut as quickly as razorvine. It grows everywhere in Pluton, and is considered a relative nuisance despite its distinguishing qualities as a medicinal herb and alchemical ingredient (mostly employed by the hags).
Whenever a lie is spoken in the presence of lying nettle, the plant instantly begins to grow. For every additional lie told, the plant grows a little more. This process is far too slow to be used offensively, but can--when planted surreptiously upon the borders of a town--quickly overrun a village and render it uninhabitable in only a matter of weeks (or days, if the town is particularly deceitful). Seeds can be used to detect lies by monitoring their movement when speaking to someone else (the seed will start to sprout if a lie is told in its presence). And finally, their most insidious use is employed by night hags wanting to teach a certain someone a lesson they won't soon forget.
If a target ingests a lying nettle seed, they will feel nothing at first--but for every lie they tell, the seed will begin to grow. If no more than two lies are told within the first day, the seed dies from malnourishment and passes harmlessly through the target's system; however, if three or more lies are told, the seed manages to grapple on to the target and will begin its incubation period.
Every time after this point that the target tells a lie, they take 1d6 points of internal damage. This damage cannot be mitigated through use of hardness or DR. However, creatures who are immune to critical damage (constructs, oozes, and undead) are unaffected by this process. The only way to remove the seed at this point is through a Remove Disease spell or something greater.
The seed only considers something a lie if the speaker is aware that what they're telling is an untruth--in other words, their purpose is to deceive.
Bloody Knives
A Bloody Knife is, as its name suggests, a knife with dried blood on it. With two locks of hair (or fingernail clippings, or full body parts), one from the "victim", the other from the "killer", the person who holds it can, as long as they hold the knife, create the impression that this was the weapon the "killer" used to murder the "victim" (the impression will even stand up to Detect Lies and any magical clairvoyance that is used upon the blade or within 100' of the Knife). If a Bluff roll is called for to create said impression, the character gets a +10 bonus to that roll. The blood will always appear to be the appropriate color and composition for the "victim". Note in addition that the "victim" need not be dead, and the "killer" need not be alive, nor need you refrain from naming yourself as either.
Thanks
Luc "Obvious Uses" French
I think you mean left it in eternal light. Just a minor typo
A minor typo with a major change in meaning 
Thanks for pointing out!
I absolutely love the Lying Nettles idea (even if there are far too many thorny vines on the planes these days). The first thing that came to mind when I read it, though, was a wooden golem made from the wood of a Lying Nettles plant. Cricket not included.
I'm a tad confused as how Bloody Knives would work, but it seems to have the potential of a really useful item in a very particular situation.
And for the eternal night/light thing - somebody now has to write up a location in Radiance that is, indeed, made of eternal night, with only a field of stars as the only evidence of it actually still being on Radiance.
Hades' Coins
As one of the primary gods of the underworld, Hades claims ownership of all the treasures beneath the earth. While this is hotly contested by many entities, it is undeniable that his vaults contain treasures beyond mortal imagination. Over all the eons, only one thief is known to have successfully breached the vault, and even she only made off with a sack containing a few thousand gold coins. Hades was not amused.
Hades' Coins look much like any other gold coin except that they are a little wider, a little thicker, and are made of pure unalloyed gold. One side bears a mark of Hades' face in the classical Greek style, and the other a styalized version of Cerberus his guardian. The coins radiate no aura of magic, but regardless they are invulnerable to any form of damage or destruction.
Anyone who spends one of Hades' Coins or accepts one in exchange for goods or services is immediately marked by a wasting curse. Starting 3 days after the date of use, the marked person's skin begins to develop a yellowish-gold hue and flake off in large patches. This will become more and more severe over proceeding weeks, dealing one point of ability drain to both Dex and Con every other day, until the cursed person dies, whereupon their soul goes directly to the realm of Hades, regardless of their alignment or religion. The drain is permanant and cannot be slowed down or prevented by any non-divine level of power.
The only way to end and reverse the curse is to personally deliver twice the number of Hades' Coins that the person used to Hades' domain on Pluton. This is tricky, as buying or trading for the coins will simply count as more coins dealt with. The only safe way to gather enough coins is to either find them or steal them. While there were once several thousand Hades' Coins working their way around the Multiverse, there are now only about 150 left. Once the required number of coins are returned to any servant of Hades in his realm, the ability drain becomes ability damage and will heal over time or by magical means.
Murder's Blessing
Another item that is a quite tempting for PCs to use until they realize that they might have made a terrible mistake.
The Murder's Blessing is a dark amulet that seems to have been crafted out of the souls of unrepelent murderers. The amulet is not a sentinent being per-se, but it does want to be equiped and used and will grant a +5 bonus to anyone trying to identify it. The amulet grants a +4 A.C. bonus, but at a terrible cost.
The wearer of the amulet must kill a living sentinent creature every single day. The amulet doesn't care about alignment, just as long as the wearer kills something. A holy paladin and a evil goblin chief will both satisfy the amulet. If the wearer doesn't kill a living being, they take 1d6 damage for every day since they last killed someone.
While killing doesn't tend to be much of a problem in dungeons, such rampant killing is far more difficult to maintain in cities and less-populated areas. The amulet will also refuse to store "credit", killing ten people in one day will not give you a free pass the next.
Unequiping the amulet is no easy task. The wearer must make a will save (DC: 20 + 1 for every day the wearer has been wearing the amulet) or find themselves unable to take off the amulet. A Remove Curse spell cast on a battlefield will also remove the amulet, and of course the night hags have their own ways of dealing with the amulet for truly desperate PCs.
Mechanically? Well, they give a +10 to Bluff in the circumstance mentioned, and short out Detect Lies and all forms of magical clairvoyance within it's narrow sphere. For example, if somebody were to look into a crystal ball, they would see a dead body in keeping with the Knife-wielder's lies, even if the person is alive.
In terms of when to use it? Besides the frame up possibilities, and the cover-your-ass possibilities, there's also the possibility of claiming you killed an enemy of the person you're speaking to. (It is not recommended that you have any further dealings with them after using the Bloody Knife, though.)
Admittedly, the mechanics are the weak part of this idea.
As a new idea:
The Mask of the Grey Death (Artifact)
Anyone wearing this cursed artifact is, for the duration that they wear it, unaffected by disease, which is a good thing for them, since the Mask infects anybody touching it with a wide variety of diseases, which can spread by all the usual means.
Note that "unaffected" does not mean "immune"; the wearer will still have whatever diseases they had, or are infected with, but will not suffer any ill (or beneficial) effect for having them.
Thanks
Luc "Again with the unsubtle subtlety" French
Damnation Tattoo: A magic tatoo instigated by the fiends and sold through night hag funded schools as magic enhancing tatoos on various prime worlds. It is known to increase one magical resistance and intellect. However if the mark is still on the skin of the unlucky sod when he dies. His soul is immediately redirected and dragged screaming to the lower planes of his particular law/chaos alignment. There their souls are ensnared and twisted into larvae for fiendish fun and profit.
Hordling Skin Shirt
Hordlings are tough to kill sometimes. In certain situations it's really difficult to tell a hordling corpse from a live hordling. Someone somewhere got the bright idea to take advantage of this in a quite morbid fashion. Stripping the skin from a freshly killed hordling of at least one size category larger than the wearer and treating it with some form of preservative (oil of timelessness works wonders) and a little creative tailoring results in what can only be described as a Hordling Skin Shirt. The whole thing twitches occasionally, and if taken off may attempt to escape, but it's unwholesome liveliness does have one advantage. The shirt retains a little of its innate self preservation instinct and will try to fling the wearer out of the way of danger. This translates to a +1 Dodge bonus to AC and a +1 bonus to Reflex Saves. However, it is far from perfect. Occasionally the shirt will interpret something innoculous as a danger and try to hurl the wearer away from it, to varying degrees of success. Also, the skin's instincts seems to fade quickly and no form of preservation can make it functional for more than a week. Not that many beings would want to wear a ragged, disintegrating Hordeling skin for longer than that anyway. .
Personally I like them both. I like some of the imagery of the Gravedirt spell better, but I also like the fact that the Seed is a horror that is unleashed by the PCs in most cases. But you're right, I guess. They can probably be combined into one entry when we get around to doing this in a final draft sort of deal. I honestly wasn't even thinking of Gravedirt when I wrote this. They both rely pretty heavily on false hope that turns to revulsion over time.
One image I liked, though, was the PCs using the seed to reanimate a recently slain emeny so that they can be interrogated. Because the larva-thing has the dead's memories the information will be at least mostly correct, but when they eventually re-kill the thing imagine their surprise at the larva burst. Was that always there? Was the villain a fiend? Or a puppet? Let their imaginations run wild on a red herring.