Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Eurystheus (trisyl.), ei, m., = Εὐρυσθεύς, son of Sthenelus and grandson of Perseus, a king of Mycenae, who, at the command of Juno, imposed upon Hercules his famous twelve labors, Cic. Tusc. 2, 8, 20; Ov. H. 9, 7; 45; id. M. 9, 203; 274; Hyg. Fab. 30; acc. Eurysthea, Verg. G. 3, 4.
Hence, Eurysthēus, a, um, adj., Eurysthean, Stat. Th. 4, 304.