Relic

The technology of creating relics has been lost to the tides of time.

Discovery

It is not known who "discovered" relics, as they are as old as time. However, some cultures place more reverence on these objects than others. Frielians, in particular vanarans from the Tall Belt, are often associated with relics given the great care they place in ancestral objects to ensure that a relic can be created from objects which have significance to them.

Commonality

Relics are a lot more common than most people suspect. However, since a relic requires powerful wielders, many "relics" lie dormant or hidden.

Manufacturing

In a sense, relics create themselves. Objects of significance during a person's life tend to accumulate energy. If after their passing this relic is maintained and nourished, there is a good chance it will eventually obtain enough power to become a relic.
Importantly, relics can be anything (although the most famed ones are usually weapons and armor). From a beloved ancestor's cooking pot, to a book that has been read to exhaustion, relics have no boundaries.

In Pathfinder terms, relics are called "scaling items".

Some still remember

The mercanes of Sharim and its libraries of magical items know the process and inner workings of relics very well.
They are in fact capable of creating relics while in the Bright, a plane where souls are not funnelled towards their gods, but rather go back into the plane's pool of souls. These souls are usually reborn into whatever the plane creates.
However, for the right item or deal, a mercane is willing to siphon the energy of souls to create a relic.