The Dolmen was the name given to an obelisk-like artifact that was made by the Lanteans on the planet known as Halcyon in the Pegasus galaxy. It was created as a form of defensive measure against the Wraith.
The device possessed a single Zero Point Module that powered a powerful interference signal that interfered with the higher brain functions of the Wraith and degenerated them into savage beasts. Higher caste Wraith were capable of retaining some of their intelligence despite the devices interference signal. This was revealed to be because the device's ZPM was nearly depleted and thus the device was losing power. Wraith were noted to feel pain the closer they came to the Dolmen.
10,000 years ago during the Lantean-Wraith war, the Ancients had created the device to protect the inhabitants of Halcyon. A Wraith hive ship came to the planet for a Culling, but the Dolmen prevented the Wraith from getting at the population and forced the Wraith into battle with Ancient forces stationed in orbit. The battle left both sides crippled with the hive ship's hyperdrive damaged beyond repair. After the Ancients were forced to retreat, the Wraith on the hive ship, trapped in orbit by the Dolmen and their damaged ship began to go insane from hunger. To survive, the Wraith went into hibernation where the Dolmen had no effect on them and landed their hive ship on Halcyon on autopilot. Eventually the hive ship's commander, Scar escaped and forced Lieutenant Colonel John Sheppard to destroy the Dolmen with a precisely aimed drone weapon from a Puddle Jumper. The precision aiming destroyed the Dolmen without destroying Halcyon, but released a massive discharge of energy that caused planetary-wide side effects and irradiated the surrounding area. The Wraith were set free of the Dolmen's power, but both the Wraith captured off-world and the Wraith from the hive ship had been driven insane from its effects and became savage and uncontrollable. However, with enough time, the Wraith's minds began to reassert themselves and Teyla Emmagan believed that in a matter of days or perhaps even hours, the Wraith would return to their usual selves. Scar believed this to be possible too but thought they also needed to feed to gather the strength to do so.
After the hive ship was destroyed and the Wraith affected by the Dolmen wiped out, it was determined that the Dolmen's ZPM had been near-depletion when the Dolmen was destroyed and likely exhausted the last of its power in the discharge released by the Dolmen upon its destruction thus rendering the device of no further interest to Atlantis. (SGA: "Halcyon")