Utnapishtim

Utnapishtim (, Yutanafisshutimu; lit. "Great Pandemonium Sea Covering All Things") is one of the three known Black Arts forms created by Zetsubō Usuguraiboshi, said to be manifestation of his various negative emotions; it's part of the Megiddo Triad (, Megido Toraiado; lit. "Underworld's Three Demonic Superpowers"), a set of magics that's based on different aspects of Megiddo, the darkest region in all of the Heroic Chronicles series — heralded and feared as "the source of all known evils" by various beings. Due to this, this makes the magic one of the most dreadful of its kind overall, something its shares with its other two brethren.

Alternatively known as the "Black Chaotic Ocean of Eternal Suffering" (暗然混海の常苦, Anzenkokai no Tokoku), it allows Zetsubō to call forth a vast destructive sea of darkness — imbued with the souls of his previous victims and various evil spirits derived from the abyss itself, destroying that dare makes contact with its being. It far surpasses the scope of most Elemental Magic, subduing the everything with its nearly-vast amounts of power — making its creator practically untouchable through normal means.

Trivia

 * The term Utnapishtim is a reference to the, specifically referencing the character from which the term is derived; he's the builder of a boat named the "Preserver of Life", which helped housed the fortunate to avoid the disaster of a storm meant to wipe out most of humanity. It serves as parallel to the famed story from Christianity, which is basically the same in detail. This is rather ironic, as the magic itself is meant to cause much destruction when invoked, housing the condemned souls of the dark creatures inhabiting it and those banished to the abyss by humankind over the years.
 * The destructive potential held by this magic far exceeds the destruction caused to Fuyuki City after the events of the Fourth Holy Grail War in the anime series, Fate/Zero.