In the World of Greyhawk campaign setting for the Dungeons & Dragons roleplaying game, the Star Cairns are a series of stone warrens that were used in antiquity by Suloise mages as magical research centers. The Star Cairns is also the name of a 1998 adventure module in which players can explore the site.
The Star Cairns are located to the southeast of the Free City of Greyhawk in the foothills of the Abbor-Alz. The cairns were originally set in a "V" pointing south with the apex near the town of Ul Bakak on the border of the Bright Desert. However, due to the magical disruptions of the cataclysm that brought an end to their use, the southernmost cairn is not apparent much of the time, as it is shifting between planes.
The area was chosen by the Suloise mages due to a confluence of ley lines that enhanced the magic properties of the area, easing their research.
The Star Cairns were the brainchild of a court mage of the Great Kingdom, Murtaree, who was given a copy of Kevelli Mauk's Tome of the Scarlet Sign, written just prior to the Rain of Colorless Fire that eradicated the Suel Imperium, and which served as the manifesto for the Scarlet Brotherhood. Murtaree came across his copy of the book in 167 CY.
The book affected Murtaree immensely, and inspired him to attempt his own revenge on the Baklunish people, and he soon gathered other Suel who were of a similar disposition, and took them to the Abbor-Alz, where they built five underground bunkers across the landscape in an area that was rich in magical energy. There the mages conducted research and began the construction of a major magical weapon. Murtaree attempted, and failed to become a lich about this time and died in 174 CY.
The Cairns were devastated and abandoned in 198 CY, as a meteor strike summoned from the heavens by Lyzandred hit the Abbor-Alz, disrupting careful experiments in one cairn, ripping it from the Prime Material Plane (to return only sporadically). The surviving mages left the area, many leaving for the Great Kingdom.
Since then, the four remaining cairns have been employed by different groups of beings for different purposes. In particular, the Armor Research Cairn has been used in the past by the wealthy of Greyhawk and Hardby as a communal tomb, although the practice has by and large ceased.
The cairns are also the resting place of the components of an artifact known as the Doomheart, a weapon of great power that has the capability of casting a cone that disintegrates all in its path (and which was so dangerous to its wielder that the mages also crafted a massive juggernaut to carry the weapon). 
Weapons Research Cairn
Originally, the mages of this cairn were researching weapons of great power to use on the Baklunish. Due to quirks in the magical energy of the area, all weapons created or forged were more effective in the short run, but soon became cursed. For a time, the cairn was occupied by a party of doppelganger bandits.
Armor Research Cairn
This cairn was devoted to the creation of armor. Since the cataclysm, however, the magical energies shifted to be more beneficial for necromancy. In years past, the cairn was a popular burial site, making it a target for necromancers and grave robbers.
This cairn was used for the creation of vehicles of war, including a rumored juggernaut that would have carried a powerful magical artifact into battle. An outcast beholder recently took up residence in the cairn.
As the name suggests, this cairn was used for the creation of numerous constructs designed for combat. This included rudimentary research in intelligent golems. The research created an intelligent and genocidal iron golem programmed to kill non-Suel.
The Lost Cairn
This cairn was where the most sensitive research was conducted, including research in replicating the Rain of Colorless Fire, as well as experiments in fiend summoning. The cataclysm affected these lines of research due to a bizarre coincidence: the meteor struck just as both wizards were activating test spells. The disruption in their work at such a critical moment caused the two spells to interact, pulling the cairn into a border area between the Astral, Ethereal and Prime Material planes.
The Star Cairns had been mentioned as early as From the Ashes, which presents them as burial sites for high-level Suel mages. Carl Sargent, the sourcebook's author, seemed to point in the direction of the missing fifth cairn being a teleporter between the other four that went haywire and teleported itself. From the Ashes also suggested that the cairns were meant to be arranged in the shape of a pentagon or star. 
In 1998, the site was finally expanded as a module for Advanced Dungeons & Dragons recasting it in its current incarnation. Its module number is 9579, and was followed by Crypt of Lyzandred the Mad and The Doomgrinder.
- Brown, Anne. Player's Guide (TSR, 1998).
- Holian, Gary, Erik Mona, Sean K Reynolds, and Frederick Weining. Living Greyhawk Gazetteer (Wizards of the Coast, 2000).
- Reynolds, Sean K. Crypt of Lyzandred the Mad (TSR, 1998).