This is an event Craft Essence. It is only obtainable by participating in a time-limited event. This Craft Essence cannot be summoned. No plans have been revealed to add this Craft Essence to the permanent summoning pool, or for old events to return.
Event reward and rare boss drop from the Christmas 2015 Event. During Santa Alter for About a Week, Holy Night Sign increased the chance of getting an additional Magic Sock drop by 20%. The additional Magic Sock drop could have a different base count from the original drop. This effect was additive, stacked with the Servant bonus, and counted if your support Servant was equipped with it.