в игре Deus Ex есть квест " Talion A.D." \ правильный перевод "Талион нашей эры" или что за сокращение может быть?

A.D.или AD как вы уже и сами написали значит anno Domini - т. е. дословно "в год Господа (нашего)". Соответствует русскому обозначению "н. э." - нашей эры.
B.C. или BC- before Christ, до Христа, соответствует русскому "до н. э." - до нашей эры.

Око за око. "око за око, зуб за зуб".
Возмездие из глубины веков. (смотрите по сюжету). у AD может быть значение - informal. advancing old age.

Talion = retaliation ; in the Mosaic, Roman, and other systems of Law, the Lex talionis, or talion law, the principle of exacting compensation, ‘eye for eye, tooth for tooth’; also, the infliction of the same penalty on the accuser who failed to prove his case as would have fallen upon the accused if found guilty.

talion ▪ law Latin  lex talionis        principle developed in early Babylonian law and present in both biblical and early Roman (Roman law) law that criminals should receive as punishment precisely those injuries and damages they had inflicted upon their victims. Many early societies applied this “eye-for-an-eye (eye for an eye )” principle literally.       In ancient Palestine, injury and bodily mutilation, as well as theft, were considered private wrongs. As such, the matter was settled not by the state but between the person who inflicted the injury and the one injured, an attitude that also prevailed in early Rome. Talion was the ultimate satisfaction a plaintiff might demand but was not mandatory; the injured person could obtain satisfaction with money if he wished.       On the principle that two different persons could not have exactly the same bodily members, the Palestinian sages enacted a law by which the injured party could not demand an eye from the person who caused the loss of his eye but could demand the value of his eye. This led to the abolition of talion in Palestine. By the 5th century BC in Rome, fines known as delicts (delict) had begun to replace talion in many instances, though the concept of talion did reemerge in medieval Germany and in some areas of Scandinavia in the 17th and 18th centuries.       Until the end of the 18th century, talion provided the rationale for such corporal punishments (corporal punishment) as flogging, branding, mutilation, the stock, and the pillory. The principle still serves as a partial basis for punishments or the assessment of fines against minor offenders in some legal systems where customary law is recognized.

Принцип: talion principle - закон талиона; принцип возмездия "око за око, зуб за зуб".