From Destinypedia, the Destiny wiki
The Radial Mast is a paracausal Pyramid artifact built by the Witness and resonating with the Light, designed for the purpose to bridge a faster-than-light connection of Darkness between the Veil and the consciousness of the Traveler. It was utilized by Calus, the Disciple and the Shadow Legion in their assault on Neptune and would attempt to connect the Witness with the Veil hidden beneath the Neomuni city. However, this plan was thwarted by Rohan, who sacrificed himself in a self-detonation that destroyed the Mast.
The Radial Mast strongly resembles a larger version of the Suppressor devices utilized by the Shadow Legion throughout Neomuna, having a rectilinear box-like shape with an ornate golden armature projecting from its upper surface. Segments of the Mast's structure rotate when it is active, and when it is in the process of forming a connection with the Veil it opens up to reveal a seemingly abstract golden structure inside that resembles a more complex version of those seen within Suppressors.