Grailstone is the name of a luminous alien rock that arrived on earth during the Mesozoic era, 150 million years BCE. Brought on a medium-sized meteorite, grailstone was deposited at two sites after it broke apart in the Earth's atmosphere: On the northern edge of the British Isles, in Wick, Scotland, and in the Bosporus Strait, Istanbul, Turkey. It has many strange properties, exhibited depending on the concentration of grailstone.
The most noticeable effect of grailstone is that it accelerates and improves the body's natural healing processes, resulting in immunity to aging or sickness and rapid healing of wounds. It also modifies the metabolism of the affected organism, reducing the need for food. Finally, the organism in question gains the ability to readily adapt to changing environmental conditions, through cellular mutation. The effect is most readily noticeable in wild animals that came under the effect of grailstone, such as eugenes, theos, and leos; ancient dinosaurs that survived the extinction of their kind thanks to their habitats below ground and rapidly adapted to their new environment. Humans are affected as well, as discovered by the Templar guardians, whose eyesight rapidly adapted to the darkness of Aeternis.
Other effects are more subtle and less beneficial, as grailstone affects the mind as well as the body. Depending on the person, it might heighten intelligence and the capacity to learn. It might also increase aggression, anxiety, and otherwise exacerbate any demons a person might possess. Ultimately, it may lead to a variety of psychological disorders: Delusions, schizophrenia, paranoia, and other. As time goes by and exposure to grailstone in its raw form accumulates, the body might be affected as well, as is the case with Tallum Broadhand: A bona fide giant, permanently bound to the armor and possessed of unnatural musculature far beyond anything a human should - or can - have. On the opposite end of the spectrum is Lurka, a particularly affected ancient Spaniard, reduced to a hunched-over, creeping monstrosity.
In extreme cases, grailstone can also affect death. At its most basic, grailstone can simply prevent the affected organism from expiring - unless mauled, incinerated, or otherwise mutilated, the body will spontaneously regenerate and come back to life. The effect can be deliberately exploited with malicious intent: Malik's experimentation with grailstone led to the creation of an undead monstrosity out of Sir Claude Lancet, impossible to kill.
Even irreparable damage to the body doesn't preclude revival (resurrection, if you will), as grailstone will preserve the consciousness of the victim and make them return as a ghostly apparition. The longer this "ghost" remains in Aeternis, the more visible it will become, as is the case with long-dead Vincent the Innocent and Niamh. Those who died recently will typically be invisible, like Colin Scott. In general, the more traumatic the death, the greater the chance of one becoming a ghost.
The effects of grailstone can be tied to the distortion field it creates, which affects more than just metabolism. It also causes time dilation: As the concentration of grailstone increases, the slower time seems to pass. 
Grailstone does not have to be ingested for it to have an effect. Mere proximity suffices for the effects to manifest, taking about six years within Aeternis, even if the grailstone is dispersed. As the concentration of grailstone increases, the effects become more pronounced. The Holy Grail is the largest known piece of grailstone, making it the most powerful manifestation of the alien rock - and is so powerful that it affects any sort of liquid placed within it, imbuing it with a diluted form of grailstone benefits. However, even in its diluted form, the Grail can heal even the most grevious of wounds, remove any kind of disease, and effectively halt aging. Anyone seeking this benefit needs to drink from the cup regularly - from several weeks to every day. The Priory at New Jerusalem seems to vary the frequency of drinking depending on the social standing of the drinker.
Such concentrations of grailstone are rare, however. It took 600 years for the Knights to excavate 8 kilograms of grailstone from the caves beneath Aeternis, with the amount lost to Malik following the purge of his court.
Notably, continued exposure to grailstone is necessary for its effects to apply. Anyone removed from the distortion field will inevitably suffer, the severity of the suffering depending on the amount of time spent within the field. Sir Guy of Bramley, for instance, after entering the outside world after 600 years of isolation perished within months, suffering accelerated aging. Geffrye of Bayeux, on the other hand, would rapidly turn to ash after being deprived of grailstone, after nearly 900 years of exposure.
- Design document: " During the Mesozoic era, a medium-sized meteorite enters the Earth's atmosphere. As it blazes a fiery path towards the Earth's surface it breaks into two roughly equal segments. One impacts with an area that will one day be known as Istanbul, Turkey - the other on a stretch of coastline that will one day be known as Dunstanburgh, Northumberland, England. The Istanbul meteorite fragments on impact and with such force that it peppers the soil to a depth of up to 60 meters underground. The Instanbul site does not come into this story. The Dunstanburgh segment however, survives the initial impact but fragments deep underground, scattering itself into an existing and extensive cave system. Time passes. Such creatures that blunder or find their way into these deep caves are nurtured and changed by the strange meteoric rock that surrounds them; they cease to age, do not sicken, and heal rapidly from their wounds. Their metabolic need for food decreases and they develop the capability to evolve rapidly to meet ne environmental demands. Animals that enter this exotic, subterranean zoo tend to stay and a complicated food chain evolves. Those that later leave quickly sicken and age - they return or die. Many millenia go by."
- Letter to the Grand Master, 1220
- Letter to the Grand Master, 1259
- Malik's log, undated
- MS-2: "DNASCAN - human species detected - extreme cellular mutation detected - catalyst unknown - subject is heavily armoured in mediaeval banded armour"
- Lurka's appearance
- The Holy Thief can survive death in at least two instances of the game, one mandatory for its completion. Edgar the Axe will also revive after being killed at the minehead.
- Entry 1502
- All of the mentioned ghosts' appearance.
- Design document: "Grailstone has many profound and strange effects upon living metabolic systems. These effects occur wherever Grailstone exists in any quantity (asis the case within the confines of Aeternis) and gradually increases with the passing of time.
One such effect is to drastically slow down all metabolic processes, An individual is not even aware of this slow-down because the other living things around him have been affected in exactly the same way, so that comparatively speaking everything appears to be normal. If this individual, however, were able to see the outside world, he would be dismayed to see it flying by at an extraordinary rate. These fields of metabolic distortion vary throughout Aeternis, but the present-time, general rule of thumb is one hour underground equals one month above ground. Hence a gameplay of twelve hours would mean that one-year will have passed in real time.
Another bizarre aspect of this distortion is that it seems to affect mechanical devices such as timepieces in exactly the same way, making them also conform to local time-fields". Like so many other aspects of Grailstone the reason for this phenomenon is simply not known."
- Jack Sinclair: "Oh, what can I say. He's become just like the rest. This place drives you crazy. It warps and twists you. As far as I can tell it takes about six years for the effects to be felt. (bows his head) And it's happening to me. My minds OK so far I think, but my body's starting to change. (looks up again) But I tell ye this, I'll find a way out before I end up like them, God help me, yes I will"
- All the characters in Aeternis are affected by the grailstone-rich environment.
- Entry 1501
- Design document: "In Aeternis the Grailstone was proving to have other unforeseen, long-term properties. Firstly, it created a gradually increasing slow-down field that affects all living metabolisms around it. Secondly, it has profound long-term effects upon the human psyche, promoting mental adaptation and growth which can be either positive or negative depending upon the individual concerned.
For some of the knights these effects were beneficial, for the rest they were catastrophically negative.
Gradually, a huge rift appeared within the Brotherhood, largely orchestrated by one knight named Mallik. The crisis of 1753 pushed the pressure between these two uneven factions to a bloody breaking points; it started when outer parts of the temple, which had fallen into disrepair, suddenly cracked and opened, giving the animals below the access to Aeternis they had long been waiting for. They literally flooded into the empty corridors and chambers, happy to have at last escaped the crowded and perilous conditions below.
Frantically, the Guardians tried to find and block the cracks, but the animals kept flooding in from other openings. Tobias (the Guardian Commander-in-Chief) manged to secure the situation but it was too late, the empty rooms and corridors of Aeternis had new and dangerous tenants - and they were there ot stay. Tobias had managed to intervene quickly enough however to stop the larger predators from getting through, but in the melee one of the knights, Claude Lancet, was killed by a dinosaur-like animal known as a Theo.
While Tobias was absorbed in controlling this disaster, a new one loomed, for each faction within the brotherhood blamed the other for the breach - two knights died in the fight that followed and a third, Richard of Bramley was forced to flee the temple. Tobias was again too late to stop the disaster, but he managed to pacify the remaining knights.
Meanwhile Mallik, who had been instrumental in the killings, decided it was time to also leave the temple - but not empty handed - he would take the Grail! However, he had no idea of Tobias's abilities and was blocked by a wall of flame. He escaped instead with the entire store of 8 kg of extracted Grail stone - 600 years of work - and Tobias, by now totally exhausted form his exertions, was powerless to stop him.
Richard of Bramley, removed from the Grailstone, died shortly afterwards above ground, but not before leaving clues that would one day lead to the coming of Raptors."