Divine Stone of Returning Soul

Divine Stone of Returning Soul (還魂の聖晶石 (かんこん せいしょうせき)) is a one of a kind item in «Sword Art Online» that can be used to revive a recently fallen player. The only way known to obtain it was to fight Nicholas The Renegade, the Christmas event boss, during the Christmas Event and defeat him.

Activation
The item can be activated on the player's shortcut menu or by holding the materialized item and shouting «Revive ... Player Name». The effect will only work during the time frame between death of the player to the disappearance of item's effect light, which is approximately ten seconds.

Red-nosed Reindeer
A month before Christmas, all the NPCs on all floors started talking about a Christmas event. Two weeks before the event, Kirito heard from an NPC that one of the treasures that the Christmas event boss carried was a revival item. After hearing this, Kirito started leveling like crazy in preparation for the event, having hope that if the item really did exist, he would have a chance to revive Sachi. On the day of the event, after reaching level 70, he equipped all his best items and withdrew all of his potions and crystals from storage to face the boss solo. However, after defeating the boss and receiving the revival item, he learned about the ten second rule. Disappointed and depressed, he gave the item to Klein, much to his friend's surprise.

Trivia

 * It is unknown if Klein used this item or not.
 * It is unknown if the item would work, as Kayaba said in the Light Novel that all forms of revival are disabled.

Piedra Divina del Alma Retornante