Catalyzes the conversion of hercynylcysteine sulfoxide to ergothioneine by cleaving the cysteine residue at the sulfur atom, the last step in the biosynthesis pathway of ergothioneine. AH2 + H(+) + S-(hercyn-2-yl)-L-cysteine S-oxide = A + ergothioneine + NH4(+) + pyruvate Amino-acid biosynthesis; ergothioneine biosynthesis. Shows substantial decrease in ergothioneine, accompanied by accumulation of its immediate precursor, hercynylcysteine sulfoxide. Belongs to the class-V pyridoxal-phosphate-dependent aminotransferase family. EgtE subfamily.