Void seeds are unlocked with the research Eldritch Epiphany which is automatically unlocked when you accumulate enough warp and is the first research in the eldritch tab.
Void seeds are used to make Voidmetal. They are also dropped by Crimson Cultists.
Contrary to the name, you can't plant them.
Crafting:[]
Crucible:
Wheat seeds into a crucible containing 2x Alienis Essentia, 8x Tenebrae and 8x Vacuos.