Pigsty

M was a virtual representation of one of Kassandra's genetic memories, relived by Layla Hassan through the portable Animus HR-8.5.

Riddle
''Sometimes soldiers can be pigs! Actually, if you follow the road going east out of Kreusis Port, there lie pits full of bloody pigs. You can find me on an altar where a sacrificial pig is being prepared.''

Outcome
Kassandra found and recovered the treasure.

Reference

 * Assassin's Creed: Odyssey