Unamed User (Deceased) Yakan Tsuki (Presumed)
Empty Vessel (空の容器 Sora no yōki) Is a caster type of magic that wasnt used for more than 100, it is one of the most powerfull magic spells in existense and one that is nearly impossible to learn and use for it can risk the life of the user and get him killed.
The spell Empty Vessel requires extremely high magical power to use it, because just about every person that tried to use that spell had died by losing his entire magical power or because it is too powerfull, only one person was able of using that spell but that was 100 years ago and the user of the magic had died after using it for a few hours, once using the spell an enormous amount of darkness flows from th user and takes the shape of a human body but with a few missing parts such as a hand, an arm or a leg but that is fixed once the darkness takes elemental materials that are within the area, the darkness takes many kinds of materials such as trees, rocks, water, plants or even fire then those materials take the form of the missing body parts and get fused into one dark giant made from darkness and fused with other materials and the user can use the dark giant for only 2 hours.
creating the giant takes 10 hours for it to be complete but the dark giant can be "Perfect" if the user is wiling to take 20 hours to create the dark giant making it stronger and bigger and with no time limit but can be used for as long as the user is either alive or still has enough magical power to use keep the dark giant stable, so far no one had been able to master that spell again since the last unknown user from 100 years ago.
If the user is able of using the "Perfect" dark giant then he will only be able to use it for as long as his magic power lasses after his magic power runs out there are two things that could happen, The first thing is the user dying from a large loss of magic power, The second is the user falling into a deep coma for a very long time which can be for years.
If the user wakes up from his coma his magic power is restored yet he feels very exhausted because of not moving a muscle for a long time, He also feels very hungry.