The casting of this spell creates a fiendish and paradoxical magical trap: the illusion of a stairway that appears to ascend (or descend at the caster's option) forever. That is, it goes up 40 feet and then make a 90-degree turn to the right (or to the left if descending); ascends another 40-feet and makes another 90-degree turn to the right; goes up another 40 feet and makes another 90-degree turn to the right, and so on, and so on in an endless loop. Turning about to go in the other direction has the same effect in reverse, with victims ascending or descending forever without ever finding a way out. Those attempting to mark their path, by leaving objects or markings in their wake, quickly discover that after making four 90-degree turns they end up exactly where they started, finding any objects or markings they left. This is in spite of the fact that they are definitely going up (or down) quite steeply on each flight of stairs and should be getting higher and higher (or lower and lower).
Escher's ascending and descending can be cast at the base of an existing stairway, at a door, archway, or other opening, or even in mid-air, in which case it creates an obviously magical door beyond which exists the spell's twisted reality. The caster can enter and leave the area of effect at will, by calling for a doorway which exists at the same place it entered, but anyone else who takes even a single step inside is caught. Those watching from outside the trap will still be able to see anyone inside until he/she either goes past the first turn, or attempts to descend. Likewise anyone inside the trap will continue to see the entrance/exit until he/she takes the first turn or attempts to turn back.
The caster can choose to release those caught in his trap at any time, but otherwise the only way to escape from an Escher's ascending and descending trap is by use of magic or psionics capable of travel across the planes such as teleport without error, plane shift, world walk or similar effect. A detect traps spell will detect the foot of the stairway as a trap, and a dwarf who makes a detect stonework trap or a thief who makes a detect traps roll will realize that there is definitely something unnatural about the stairway. Detect magic cast by a wizard who makes his percentage roll to know school will perceive strong illusion and alteration magic. A dispel magic spell cast at the ascending and descending effect from the outside has the normal chance of dispelling it, but has no effect if cast from inside the trap. Anyone protected by an anti-magic sphere or protection from magic scroll will find themselves unable to enter the area of effect, physically repelled from it, but if cast or read once inside the trap these spells have no effect on the trap itself (but otherwise function normally). A Mordenkainen's disjunction spell will end the ascending and descending effect from the inside, but it will also dump anyone inside into the deep Astral Plane, with only those who were touching flesh to flesh when the spell was cast ending up near each other (others could be anywhere from a few miles to the equivalent of weeks of Astral travel away).
At the end of the spell's duration, or if the Escher's ascending and descending is dispelled from the outside, anyone and anythging inside immediately emerges at the point of entry, being forcefully expelled as the effect collapses.
Escher's ascending and descending is a spell effect that can be made permanent via a permanency spell. The material component is an unbroken twisted loop make from a flat ribbon of steel.
Notes:
PC's trapped in an Escher's Ascending and Descending are likely to come up with desparate and sometimes ingenious ploys for trying to affect an escape. I cannot hope to answer every possible contingency, but here are a few considerations:
Using extra-dimensional magic such as a *rope trick* spell, *Morden- kainen's magnificent mansion* and co. will not allow the characters to escape the trap, however if they are still inside these effects when the duration of the Escher's spell ends, they will be dumped into the astral plane as if the spell had been disjoined.
Characters with magic items to burn might try throwing a *bag of holding* into a *portable hole*. This will open a rift into the astral plane through which everyone inside the Escher's trap will be sucked. It will destroy both magic items and leave the characters floating, unconscious until or unless rescued (or captured) by kindly travellers. By contrast, placing a *portable hole* inside a *bag of holding* within an Escher's ascending and descending trap causes an explosion as described in the DMG and opens a rift into the deep Ethereal Plane, where anyone inside the trap will be dumped.
A *limited wish* spell will open the door and allow PCs to exit the trap normally, and, of course, a *wish* will transport the characters almost anywhere. (Although if incorrectly worded it might still dump them into the Astral Plane.)
A character who attempts to use the *dimension door* spell to purposefully occupy the same space as the apparently solid walls of the trap (in order to by cast into the Astral Plane as per the spell's description) will appear at some other point within the trap. He/she is allowed a saving throw verses death magic to avoid being struck unconscious.
Spells such as *phase door*, *passwall* and the like open a tunnel or portal form one part of the trap to another, determined at random.
A *worldwalk* spell, *gate*, *amulet of the planes*, *planeshift*, or similar spell will function normally within the effect. *Oil of Etherrealness* (which only allows travel to the border ethereal) will allow the user to walk through the walls surrounding the trap, only to emerge in another part of the trap, as per passwall and phase door.
Magic resistant creatures or characters who somehow can make use of magic resistance, can make one attempt to exit the trap using their resistance. Success means that they (but no one else) is throw out of the trap. Failure means that the resistant creature is stuck like everyone else.
For purposes of conjuration/summoning spells, the trap should be considered to have access to all of the planes ascesible from the prime material. It exists, in fact, as a quasi-planar rupture (its own universe in effect) "between" the astral and ethereal planes.
There are many other possibilites (as a last resort player characters have been known to start shouting insults at powerful extra-planar beings and well known archmages--you never know when they might take heed to the sound of their name!) In any case this spell is powerful magic and only the most powerful magic or most desparate tactics should have a chance of defeating it.
Besides Escher, the Mistmaster is known to have access to this spell. Others may as well, particularly high level illusionists of neutral or chaotic alignment. When Escher finds character in one of his traps, he generally sends in a project image to question characters about their motivations for invading his privacy (his abode is located atop a sheer spire in the Spine of the World mountains). If they don't have good answers he will either disjoin the trap from the inside (dumping the party into the Astral Plane) or attempt to slay them outright if he has reason to believe that they are evil.