Eiryn Muss is a secondary character in Ian Irvine's The Well of Echoes quartet. In the beginning of Geomancer, he is a perverted halfwit drunk who works at the manufactory, growing fungus for an extremely low wage. At one point, Irisis begins to suspect that he is smarter than he looks and is proved right when, at the end of the novel, it is revealed that this was all a brilliant act and that he was in fact the other prober at the manufactory, working directly for Xervish Flydd. He performs several other missions for him during the Well of Echoes series.
During Chimaera, his past and motives are revealed. He was originally a young apprentice at Nennifer and trained with one of the mancers there. However, the mancer wanted a specimen rather than an apprentice and used Muss in an experiment, trying to combine him with a Lyrinx to create a Chimaera, a hybrid that could be stronger, bred faster and controlled easier than humans to use on the battlefield against the council's enemies. The experiment left Muss in constant pain and mentally at war with himself but no stronger than before and without any reproductive organs. Since he was a failed experiement, the mancer threw him out of Nennifer. However, Muss found he had gained two qualities from the experiment; he could flesh form better than any Lyrinx and he could apply the talent to his own body to take any shape roughly his own size, though the process caused him a lot of pain until he could control it better. He re-entered Nennifer in disguise and learned more and more about the Secret Art and how to reverse the experiment. However, since he was only an average mancer in all respects except flesh forming, he needed a special material, Nihilium, to imprint the spell onto first. Fearing the Council too much to try and influence their actions, he instead became an exceptional spy for Xervish Flydd. He is the person who sabotages Tiaan's crystals and drugs her at the manufactory to undermine Flyyd and make Jal-Nish scrutator instead. Finally, he manipulated Flydd into destroying the node at Snizort. However, he failed to acquire the Nihilium created there and was captured by Flydd's group trying to retrieve the material at Nennifer.