At first glance this is nothing more than a small, flat, and grey rock with a spiral etched into the most flat side. The dull grey color belies it's true purpose: creating magical portals. The infamous mage Torpin 'The Dubious' of Longshanks created this magical item ages ago for a single purpose and once he was done, the stone passed into obscurity. Over time, some of the magic has faded from the stone, resulting in a less than trustworthy portal operation.
When through onto a flat surface this stone will disappear and will create a stone spiral staircase stretching in the direction perpendicular to the surface on which it is thrown. If thrown on a floor, the staircase will stretch downward. If thrown on a wall, the staircase will stretch horizontal to the ground. If thrown against a ceiling, it will create a stairway that reach upwards. Whatever the stone is thrown against must be a substantial solid object with at least ten square feet of surface. Throwing it into water will simply cause it to sink to the bottom and create the stairs under the water (and the ensuing chaos that will cause). The effect will remain in place for d66 minutes, at which point the stairs - and anything on them - will be destroyed. All that will remain is the simple stone, exactly where it was thrown. There will be no indication of the stairs about to close, one moment they will be there, and the next they will cease to exist.
Stairs will reach in the direction they are created for d6x100 feet, any opening the stairs crosses paths with will cause an archway to appear. If a character is near an opening when the stairs begin to collapse, they are allowed a saving throw to leap clear. All other items or creatures, regardless of level or power are simply destroyed. To the right buyer, this stone would easily fetch 500 coin.
Dangerous add on:
Every time this is activated the DM should roll a d6, on a 1, something interesting happens. Instead of opening a simple staircase, the stairs end after two complete turns, coming to an archway that opens to a random plane of existence. This effect will last only half of the normal duration. If opened to a plane, the ruler of that plane may close this stairwell as they wish.