This page details how to complete the Le Vieil Arbre mystery in Paris, Francia, Siege of Paris DLC.
How to Complete the Le Vieil Arbre Altar Offering
This mystery can be found in the northern part of Paris, just northeast of the Clever Fox Tavern. To complete the mystery you’ll need to present 100 silver and 2 Animal Hearts at the altar. You’ll most likely have an abundant supply of both at this stage in the game, but if not you can kill essentially any animal for the hearts. If you don’t want to travel far, you can even kill the cats, dogs, and chickens nearby for a heart. If you’re roleplaying a monster, that is.
Once you have the necessary items, all you need to do is interact with the altar to complete the mystery.