It was made many centuries ago, and although the reason is now a bit hit and myth, the fables are mainly truthful, in that the great mage Edward the Purple (who got his name because he was the royal mage of the King of the time) made it when his beloved wife Tina died of a virulent form of the Sun Plague.
Using all his magical skills and power he fashioned a portal to the other side with many sigals and runes and pentagrams around it to ward off the demons and the undead, and he brought his loved one back, but as he clasped her to him the portal claimed his life and it is said, his very soul, and he sank dead to the ground.
Over the centuries and a bit of trial and sometimes dangerous error the secrets of the portal became known. First, for every person it restored to life it took a life in it’s turn, be it the life of a criminal that was being sacraficed to it or the life of a willing person or just of someone who got too close, sucking out the life and the very soul.
Magic cast anywhere near it tended to react in strange and unpedictable ways and so all casting of spells and charms within a hundered feet of it were banned for everyone’s safety.
To bring a person back the body had to be reasonablely fresh and not decaying or cremated, although minor rotting, when the veins and arteries went green, would not block the spell. The person had to have died violently and to be below the age of fifty.
Finally when activated the portal sucked up a huge amount of magical energy from the ley line on which it sat, so that to use it too long and too often would destroy the kingdom’s main clathrate and with it all the magic in the area, so it was decided that it could only be used on Middsummer’s Eve.
(I have left it up to the GM to describe what the portal looks like so it could be different for each world.)
The Great Escape
The PCs are escorting a dangerous criminal to the portal for him to face execution, but his friends have other ideas and want to set him free
The PCs are transporting a dead body to the portal and it is a race against time before the body decomposes too much to be raised
A Disaster Waiting To Happen
The pentagrams are failing and unless the portal is destroyed soon, evil will leak through and endanger the whole region. But the Mayor makes a lot of money from the portal from people who pay to have their loved ones restored and will not take kindly to the PCs trying to destroy it, and neither will the city guards. It can be played in *real time*.
Imprisoned for Life
It could be that the portal works by sealing two souls in one body and using the two to heal the body.The soul that is the sacrafice is trapped within the body, able to see and hear and think, but has no control over the body it is in at all. If something were to go wrong with the portal then these imprisoned souls would have the chance perhaps to seize control of their prisons, and how sane they would be would vary widely.