Diabolical Extremities

Diabolical Extremities (腐純化式典の堕悪魔祓い魔導士 (ダイヤボリカ·エクストレミティース) Daiyaborikar Ekusutoremitīsu; lit. "Tainted Purification Ceremony of the Fallen Exorcist Mage"), is the ultimate technique belonging to a Devil Slayer — being both the embodiment & pinnacle of the Devil Slayer Magic style they practice. Rumored to have the power to utterly throw nature into chaos, it's a spell that can kill a devil without fail — regardless of their given nature and/or capabilities; this gave it the title of "Ultimate-Devil-Slaying Tool" (極滅悪用器, Kyometsuaku Youki).

When compared to other forms of Slayer Magic, it's said that this spell is considered to be the top-tier amongst its kind — outright surpassing the Secret Arts belonging to both Dragon and God Slayers alike. Ironically, the use of this technique can be seen as a form of heresy: the user's reliance on demonic traits to utilize an immense power in order to slay the very thing the magic was designed for. Thus, this technique seen as a taboo amongst Devil Slayers, garnering another title "Contradictory Bane of the Exorcist" (悪魔祓いの抵触害, Akumaharai no Teishokugai). It's only to be used when one wants to throw willingly away the title of "Devil Slayer" in order to accomplish their cause at the risk of becoming a disgrace to their kind.

The technique can only be learned by reading the final page of the book pertaining to the user's Devil Slayer style. While normally illegible, it reveals itself if said user mastered their respective style to the best of their ability while fulfilling the proper conditions pertaining to said ultimate technique. It should be noted that it can be only used while one is in Devil Synchronization; this is due to the sheer power it possesses being too immense on the user while in their base form.

Overview
"What I'm about to show you is something no other has seen. This spell is the true form of my Devil Slayer Magic. It serves as testament to my mastery over the style, being unmatched by any other. With it, all things before me fall into ruin; even those of equal power can barely stand up to it, let alone fight against it. The insurmountable surge of power that emanates from this spell is enough completely disrupt and halt the very energies of nature itself, causing various abnormalities. One... can call it an "Act of the Devil", something which seems to be ironically fitting. Now, I should warn you.... There have been none who faced this spell and lived to tell the tale. And I assure you, after this you won't either. Let it begin! DIABOLICAL EXTREMITIES!"

- A Devil Slayer to an opponent before activating his ultimate move.

Trivia

 * There can only be one technique per Devil Slayer style due to the power it possesses. No exceptions.
 * This is meant to serve as a parallel to "Dragon Slayer Ultimate Art" (滅竜極義, Metsuryū Gokugi) — a concept invented by D-Dawg . All credits go to him and his awesomeness :P
 * This is based off of Extreme Magic from the anime/manga series, Magi.