Spoilers for Assassin's Creed Origins.
At the end of the game Aya says "requiescat in pace" after killing Ceasar. The line is a reference to Ezio, a character from an earlier game, who lived in Italy during the late 1400's and early 1500's.
I was wondering if those words would have been used in that context in that time period or if it's completely inapplicable and simply a reference for the sake of a reference because they just happen to be in Rome at the time.
'Requiescat in pace', or rest in peace, is most commonly associated with Christianity, and also in Judaism. There is evidence of a form of the phrase being used on a Latin inscription from around 688 AD, but the earliest known appearance of the phrase on tombstones wasn't until the 7th or 8th century AD, and really gained popularity in Christianity in the 18th century.
In a way it appears in the Jewish mourners prayer, the Kaddish, which refers to God as the creator of peace and wishes for peace for all Israel, but it is unknown how old the Kaddish is. It was likely not developed as a mourner's prayer, but instead was said at the conclusion of a Torah reading. In that case, it had no the connection of specifically wishing peace for the dead during its early intended use.
In the way that it is used in Assassin's Creed, as a short prayer over a recently deceased person, then I think that it's been put in purely as a reference to Ezio.
*References:
Robert Jefferson Breckinridge, Andrew Boyd Cross. "Antiquity of the Religion". The Baltimore literary and religious magazine. 3. p. 206.
JIWE, I, 189 (D. Noy, Jewish Inscriptions of Western Europe, Cambridge, 1993).
Student, G., 2010, A Brief History of Kaddish, http://www.torahmusings.com/2010/12/a-brief-history-of-kaddish/