The Hexagraphe is an Aeldari mapping technology that is powered by a psyker. It resembles a perfect sphere, little bigger than a child's skull, flawlessly smooth and so black that it appears to absorb light rather than reflect it.
When a psyker channels Warp energy into the sphere, the Hexagraphe will absorb it before detonating. After the detonation, the psyker will be imprinted with an extremely detailed map of the surrounfing region and all of the lifeforms it contains.
The Deathwatch were known to have used one such device on the mining planet Chiaro. The Hexagraphe had been captured from the Aeldari under unknown circumstances.
- Deathwatch (Graphic Novel) by Steve Parker