A Gehemehnet is a giant spire or obelisk built by the forces of Chaos (most notably the Word Bearers) that channels Warp energy into a planet, turning it into a Daemon World. These are built out of the remains of buildings that were in the city that formerly resided on the construction site, bonded together with a substance known as "Blood Mortar", created from the bodies of the enemy.
Actual production is on a massive scale, the entire region is transformed into a hellish landscape of forge works and factories to mass produce the materials and daemon engines required to make construction possible. The majority of the building is done by a slave labour force usually numbering in the millions, presided over by cruel overseers and assisted by demonically possessed spider-like cranes to lift the house sized blocks into place.
When completed, the tower on Tanakreg built by the Word Bearers was just over a kilometer in diameter and an estimated 50 kilometers high. 1 The daemonic forces contained within the tower are attributed to keeping it standing despite it breaking the laws of conventional physics.