Slitherine

Sorry, but I don't find any way to transmute Artefacts (Get rid of your excess artefacts by turning them into other resources or gold), and I am playing Onslaught for the second time: Could someone help me?
.

Hope this helps. I, also, was unable to find the Transmute Artifact in Onslaught. Possibly, like the Merchant, it is only available if you play and Empire hero. (Which you can now do as they added one to Onslaught a few days ago.)

Dev says they may have made a mistake on this, it is currently not available for Onslaught. Guess they will fix this in the next patch.