Regression

The entirety of this plot takes place over (maximum) 2-3 days of game time (as perceived by the PCs); perhaps only 1-2 days.

Prelude
The PCs (probably of medium to medium/high level) are currently in Tournais, a fairly large and prosperous city (not the capital) in the large nation of Givet. The party is hired by Duke Jaelend of Tournais to seek out and kill the evil mage Zarakoth, known to be practicing dark arts in the Forest of Ludenec, nearby. They are assigned an NPC mage, Corwyn Feldrew, to help them.

The party progresses to the forest and engages Zarakoth in combat. The next bit may require some GM cheating of game mechanics. Zarakoth is mortally wounded (either by spell, arrow, whatever). With his dying breath he summons a mighty spell and hurls it at you. Corwyn leaps in front of the blast, desperately chanting a counter-spell: however, he was not quick enough. Corwyn, crumples to the ground, dead: however, it seems that his counterspell was enough to halt the magic as the faint tendrils of magic that touch the rest of the party have no effect. Zarakoth is also dead.

The main plot i.e. the interesting, non-cliche bit!
As the party leave the forest they will, at some point, stop to eat. They notice that their lunch seems to have gone off - the cheese is mouldy, the fruit rotten, etc. (No magic can be detected around any of it, should they try). Probably the party will just ignore this - at any rate, they will almost certainly continue on back to Tournais.

Down the road a little way they notice a small party of riders (around 4) in armour that bears crests similar to, but slightly different to the arms of Duke Jaelend of Tournais. As soon as the riders catch sight of the party the riders charge and attack. They will not parly. Once the riders are killed, if the party inspect the bodies they will find warrants ordering the immediate death (without capture or trial) of all of them. These warrants are signed by 'Duke Zarakoth of Tournais.' If the party doesn't find these warrants it doesn't matter. If they do, however, they should start getting very suspicious and worried.

They continue, either back to Tournais or away from Tournais. If they try to go away they will hear in a village/town that Duke Zarakoth has set a cordon 20miles around Tournais to stop them from escaping. They will also be attacked by men of the duchy again (basically, provoke the PCs in to deciding they need to go back to Tournais and kill Zarakoth again).
Much more worrying though will be the following events:

- When they enter a shop and pay for something, some of the coins will be rejected by the shop keeper. Some of their coins have changed from what they were before to bearing the head of Zarakoth. Some shop keepers will only accept the 'correct' currency, others only the new currency. As the hours pass, more and more will only accept the new currency.

- The rot and decay continues. Often they might buy a loaf of bread only to find it has gone mouldy and rotted an hour later. As time passes, this might happen even in the shop itself. Knives, swords, etc. may also rust and turn to dust.

- The world itself regresses. Over the course of the couple of days in which this plot plays itself out it should regress from the current day, through to earlier periods of history (e.g. if they started in Mediaeval in our world they would go through Anglo-Saxon (in Britain), Roman, etc.) and right back to the bronze age and ultimately the stone age. Things around them will change - the houses, the people's clothes, etc. A sword will change to - from being a steel broadsword it will turn to bronze and then to a flint knife. Things with no earlier equivalent (e.g. full plate armour in the stone age) will just rust or rot and turn to dust. (For wizards - powerful 'recently developed' spells may no longer work). All this happens gradually overall, but each item changes instantly (unless it rusts, which happens gradually (but very fast). However, throughout it all, Zarakoth is always Duke/Governor/Commander/Chieftain etc. of the city.

- The characters age (not by thousands of years, but so that they would be dead by extreme old age after 2-3 days). Apply all the normal aging penalties from the system you are using and maybe even more as they become barely able to walk, bent over, etc. The other people around them will not age like this, with one exception: Zarakoth (should they find him) will have aged with them.

- All of these things happen at an ever increasing rate as the 2-3 days progress.

Culmination
Throughout it all, Zarakoth's forces hunt and pursue them. Death is real. If they just run away, then, after 2-3 days, they will die of old age (The End). They must find and kill Zarakoth. Although the regression of the world and their own aging hinders them, it also hinders him - it is probably harder to find and kill them when he is only chieftain of a stone age tribe than a Duke in a castle. If, however, they wait too long, they may be incapacitated by old age. Every time Zarakoth kills one of them (if he does), his aging is reversed a little bit.

Explanation The PCs should not know any of this until afterwards
Zarakoth's final spell did not have the effect they thought it did. Corwyn succesfully protected himself from the spell: however, the rest of them were effected (which is why, to them, he seemed to die - he actually managed to evade it). The effect of the spell was to imprison the party's minds inside the dying mind of Zarakoth, where he has recreated a pseudo-world.

Zarakoth, however, is near death - he only has seconds to live in the real world. The '2-3 days' perceived by the PCs actually takes place over around 5-15 seconds. As Zarakoth dies, his mind dies - reflected by the rot and death of the world, its regression backwards in time and the aging of the characters.

If the characters are killed in this 'Zarakoth-world' they die properly - the death of the mind means the death of the body. The same is true if they die of the greatly accelerated 'old age'. The only way to escape is if they can find and kill Zarakoth in his world: this will destroy his mind while his body is still living, allowing the PCs to escape (they will be returned to their body. All aging effects are removed). Someone in the real world killing Zarakoth's body before they escape would result in the death of the PCs (their minds are still trapped in his).

Zarakoth has not only done this for revenge, to take his killers down with him. It is also his only chance to survive. Everytime he kills (not necessarily personally) one of the PCs in his mind-world, he can absorb their strength (which is why his aging is reversed). If he kills them all, he will be strong enough to heal himself and return. This is why he is trying to kill them.

This plot was greatly inspired by the novel 'Ubik' by Philip K. Dick