The Soul Forge: a single-level branch off Gehennom that contains a smith. The smith will take two items of the same type and forge them together into one; in technical terms this destroys one of them and imbues the other with whatever object properties the destroyed one had. They cannot both be artifacts, and if one is an artifact, it will not be the one destroyed, so the properties from the other one will be transferred to the artifact.

The smith’s price is based on how many object properties the resulting item will have, increased if it will be an artifact. In dNetHack, the smith would require payment in soul coins, but in variants without those, they may just ask for gold or some other valuable. If paid in soul coins, the requested type of coin would be random but deterministic based on the object IDs of the two items.