This seed is the result of infusion of a normal seed with the rare energies of the Eldritch Monolith.
Put seeds into the Darkness Generator and wait.
This seed can be found inside of the monolith's chest. Be aware that one seed is needed to build a Darkness Generator, dont spend all of it until you have at least 1 generator to make more.
This is the principal component to make Void Ingot.