The Forge of Solus Prime refers to two objects given as a gift in tandem to the Sixth Prime: a mighty forge and hammer capable of crafting anything from anything.
With it, Solus forged the six Matrices, and she allowed Nexus to draw upon its power to create the Pacts.
The forge part of the gift was destroyed by Liege Maximo, and so only the hammer remains as the Forge of Solus Prime. The fact that a forge was even gifted to Solus has been forgotten by many religious groups.
A Primal artifact.