The Gatekeeper's Crystal was a powerful magical device, which, according to legend, had been created by a powerful lich. The truth, however, is that it originated from the Outlands, having been carried by a mysterious being called the Gatekeeper, whose sole duty was apparently to guard a crossroads of gates across the planes and spheres, preventing anyone who wished to restrict access to it from blocking the travel of others. It was stolen (with the assistance of a deity, it was said) by Iyraclea, an ice-priestess of Auril of the Great Glacier known as the Ice Queen, as part of her attempt to conquer Myth Ondath. 
It was made up of three separate shards, each of which were powerful in their own right, but devastating when brought together as a whole. When the crystal was used as a whole, particularly to destroy a mythal, its three shards separated and scattered across a vast area, almost always including other planes.
Anyone who touched any of the three shards of the crystal would know instantly the direction in which the other shards lay, although if they were not on the same plane that fact would be all that could be discerned.
When the three shards were joined, the wielder of the crystal was given the ability to create either a dead-magic zone or a wild magic zone, as well as knowledge of any gates or portals within 50 miles (and the ability to open or close them).
Bringing the crystal in contact with any creature or item that was not native to the plane where it was found would send it back to that to which it was native.
Combining the shards of the crystal in pairs manifested different effects: the first and second blocked clerics, paladins, and rangers from using their magical abilities within their range; the second and third did the same to wizards, sorcerers, and bards; while the first and third blocked undead from the Positive Energy Plane or Negative Energy Plane, if they bore such a connection.
Individually, the first shard also blocked necromantic, transmutation, and enchantment spells; the second shard blocked evocation, conjuration, and illusion spells; and the third blocked divination, abjuration, and enchantment spells.
If all three shards of the crystal were divided and carried out so as to surround an area and their powers cast, any mythal or ward which lay partly or completely inside their triangle would be destroyed in an explosion of fiery, violet light—an act which usually also collapsed any buildings and slew any creatures found in the area, as well as many miles around it, thanks to the resultant earth tremors.
Once stolen by the Ice Queen, the Gatekeeper's Crystal was used in 633 DR to destroy the defences of Myth Ondath, an act which incidentally also took out her general, the lich Vrandak the Burnished, and almost the entirety of her army.
The crystal resurfaced in 712 DR when the nycaloths Aulmpiter, Gaulguth and Malimshaer attempted to gather its shards so as to destroy the mythal of Myth Drannor. Thankfully a brave Leiran priestess sneaked into the fiends' citadel and managed to steal one of the shards; the others were lost when the Army of Darkness and Fflar's defenders slew each other and the city fell.
It was used again in 1369 DR by the Harpers, with the aid of an archmage known as the Mistmaster, to raze Hellgate Keep. One shard was found in the ruins of Hellgate Keep by Sarya Dlardrageth. Sarya found another in 1370 DR in a volcano in Avernus.
In 1371 DR the mages of Reilloch Domayr located one of the shards and kept it in Nandiyerron's Armory in Tower Reilloch. It was stolen by demonic invaders in 1374 DR and returned to Sarya, thus completing the device.
On Alturiak 16, 1374 DR, Sarya used the complete crystal to utterly dispel all of the magical protections of Nar Kerymhoarth and freed almost two thousand fey'ri imprisoned within for the past five thousand years. After that, the crystal disappeared again, but in the same year Araevin Teshurr decided to assemble the crystal one more time when it turned out that it could be used to seal the Waymeet, a powerful interplanar mythal that could create portals between any two planes, of which Sarya and Malkizid tried to gain control in Myth Drannor. The first shard was found in Nar Kerymhoarth in a lair of ophidians, who recognized it as an idol, and was stolen from there by Maresa Rost.
According to the knowledge gleaned of it over the centuries, the Gatekeeper's Crystal could be destroyed in one of three ways: by being joined together by the Magister and then sundered by Azuth, Mystra, and Savras (something which as of 1385 DR was sadly no longer possible); by placing it inside a whole and uncorrupted mythal for 1,001 years; or by closing the nexus of the planes which the Gatekeeper guarded.
- Ed Greenwood, Eric L. Boyd (1996). Volo's Guide to All Things Magical. (TSR, Inc), p. 107. ISBN 0-7869-0446-1.
- Richard Baker (August 2004). Forsaken House. (Wizards of the Coast), p. 33. ISBN 0-7869-3260-0.
- Brian R. James, Ed Greenwood (September 2007). The Grand History of the Realms. Edited by Kim Mohan, Penny Williams. (Wizards of the Coast), p. 93. ISBN 978-0-7869-4731-7.
- Richard Baker (August 2004). Forsaken House. (Wizards of the Coast), p. 40. ISBN 0-7869-3260-0.
- Ed Greenwood, Eric L. Boyd (1996). Volo's Guide to All Things Magical. (TSR, Inc), p. 106. ISBN 0-7869-0446-1.
- Ed Greenwood, Eric L. Boyd (1996). Volo's Guide to All Things Magical. (TSR, Inc), pp. 107–108. ISBN 0-7869-0446-1.
- Richard Baker (August 2004). Forsaken House. (Wizards of the Coast), pp. 55–57. ISBN 0-7869-3260-0.
- Richard Baker (June 2006). Final Gate. (Wizards of the Coast), p. 23. ISBN 0-7869-4002-6.
- Richard Baker (June 2006). Final Gate. (Wizards of the Coast), pp. 57–59. ISBN 0-7869-4002-6.
- Ed Greenwood, Eric L. Boyd (1996). Volo's Guide to All Things Magical. (TSR, Inc), p. 108. ISBN 0-7869-0446-1.