Head of Saint Denis

The Head of Saint-Denis was an ancient relic, a head-shaped lantern encasing an Apple of Eden. While fueled by the Apple, the artifact emitted blue shimmering light and possessed the power to project illusions. Following the Apple's removal, the Head of Saint-Denis became a regular lantern.

History
Stored within the Saint-Denis Temple, the lantern was used in the 12th century by the Abbot Suger to create a powerful sword known as the Eagle of Suger.

In August 1794, Arno Dorian battled a group of tomb raiders sent by Napoleon Bonaparte in order to find the artefact in the First Civilization Temple hidden beneath the Basilica of Saint-Denis. The Assassin killed their leader, Philippe Rose, and used the power of the Apple contained within the lantern to repel the remaining raiders.

Realizing the dangers of the lantern should it fall in Napoleon's hands, Arno contacted the members of the French Assassin Brotherhood, asking them to send the Apple to Al Mualim in Cairo, while Arno kept the Head of Saint-Denis itself as a memento.

Reference

 * Assassin's Creed: Unity - Dead Kings