The Warp Nexus is an artefact construct located on Titan and is at the heart of the Fortress Monastery of the Grey Knights Chapter This star-shaped chamber was constructed by Malcador the Sigillite and is one of the few remaining relics of his work. Its first usage is during the initial creation of the Grey Knights when the entire moon of Titan was launched into a safe refuge within the Warp. Following that time, the mandalas and pentagrammic sigils maintain both the Chapter Monastery as well as Titan itself from the turbulent currents of the Immaterium.
Since its initial usage, the Grey Knights had sought to realign the power of the Warp Nexus in order for it to make use of the Warp as a place of sanctuary should it ever be required. However, no Grey Knight alive holds the sorcery capable of accomplishing such a feat and thus the moon exists both in realspace as well as the Warp at the same time. As a result, the chamber containing the Warp Nexus holds cloisters of two hundred Chapter serfs that echo with canticle, intonation, prayer and chants that fuel this arcane device.