Sentence:  Said to have been martyred as a Christian legionary commander of late Roman times for having refused an imperial order to kill one in ten (that is, <WSD>"decimate</WSD>" in the Roman meaning of the word) of the soldiers of another legion which had gone into revolt... 
Meaning of the word marked with <WSD> tags:  To kill one-tenth of a group,  as a military punishment in the Roman army selected by lot, usually carried out by the surviving soldiers.