It's not very common, only growing in places in which magical battle was waged, be the battle yesterday or many centuries ago, colonising the craters torn out of the ground by a large enough magical spell. The taste is very good, like well cooked chicken, but soon after it swallows, it puts three poisons, one of them clearly magical and still not fully understood, into the body. The first stops the heart beating. The second numbs the pain of the heart attack so it goes unnoticed, but the third is the one that makes this mushroom so special and so unpleasent. It prevents the spirit from leaving the body. In effect, the body dies but the brain is kept alive. In the short term, the person or animal poisoned by this often has no idea that there is anything wrong with them. The trouble comes later, because dead bodies will soon start to decompose.
The first sign that anything is wrong might be a few flies landing on one and having to be swatted away a bit more then normal, but most people will think they are just being pestered by them. Within four to six hours, however, (longer in cold conditions) the person's muscles will first stiffen and then freeze up totally and stay that way for twelve to twenty-four hours or so. What happens next depends on where the person is when the stiffening occurs, and what the season is.
In a warm month
For those efected by this sinister fungus, if they are in some place in the wilderness or otherwise out of the way and alone, one of two things will tend to happen. Either a predator or scavanging animal will find them and feast, tearing the affected body to ribbons before rigor mortis wears off. Or flies and other insects will find the body, unless it is very cold, lay eggs all over it, and by the time the person can move again they are maggot ridden. If they were unable to cover their eyes in time, they will be totally blind, even if they have covered their eyes with something in time,their sight will be dim and will get worse every day as their eyes rot. Within a week, they will only be able to see the difference between light and dark. Their maggot ridden marbled skin will be horrible to look upon,speaking will be harder and harder to do,and they may well be mistaken for zombies or ghouls and cut down accordingly. Their mind is more resistant to decomposition, but within a matter of weeks the affected person will hallucinate and go insane as their brain goes to mush.
If they are in a city, town or village then unless they are with friends or family who understand what is happening to them, they may well face either the agony of being burnt on a pyre whilst frozen by rigor mortis, or face burial in the ground, trapping the person in a coffin under six feet of earth.Unless dug up by friends and family who hear their cries, grave robbers, or a necromancer, they will stay there, their fingers broken from scrabbling at the coffin lid,until their mind rots and they die for real. Should they be sent to the morgue or the undertaker's parlour instead, they have a good chance that their body will be protected from the flies and the rigor mortis will wear off before they can be buried or cremated. If they can escape from there, or be released by someone who thinks they are alive, they can shoo away insects that would land on them, and keep their sight a little longer by keeping their eyes wet. With care, they can keep their eyesight for a rather long time,at least until their brain decomposes and they cease to take care of themselves.However, they will still look undead, and will face major problems.
As well as those who would kill them for being walking corpses,and the constant danger of insect infestation, they will have no protection in criminal or civil law. In some cases they may be legally considered pests;at best, they are outlaws.Since they are dead, their property is no longer theirs and they cannot be murdered since they are dead in law. They can be evicted and stripped of their bank accounts and passports if such things exist in the world where this mushroom exists.
In a cold month
In a cold month, or if they can reach somewhere cold and fly free in time becausethey know what is happening to them, those that survive the rigor mortis problem without being eaten, cremated, or buried, have a fair chance of mummification of a sort, a far better fate. If they can avoid or prevent rotting, they will stay saneand perhaps even be able to pass as one of the living. Should they be uncovered as undead however, they will still face a lot of prejudice and the legal problems described above. Wounds will not heal;eyes will glaze unless tended with care,and fungal infections in this state can be a disaster,rapidly covering the body unless kept at bay by magic or drugs.