I'm sorry to ask, but does anyone know where this is located? I'm very interested in this weapon art. Thank you in advance if anyone happens to know!

Seems like this weapon art is only available on the Godskin Peeler twinblade.

The Ashes of War required to put this ability onto weapons is dropped by slaying the Godskin Duo boss in Crumbling Farum Azula

it also drops form a boss called the godskin duo

Any chance this works on staffs such as Prince of death

Any chance it works on staffs

Black Flame is associated with Godslayer incantations. So does this benefit from the Godslayer's Seal? Is that even possible?

This Ashe of War comes from the Godskin Duo boss. It is a fairly hard boss by the way, a lot of people struggle with this one and can't get past without a level 10 mimic or other good teammates. It is kind of a bad skill, but it has niche uses. It is really easy to get knocked out of it before you get any damage off in basically any boss fight. However, it's pretty sick and fashion wise it looks really good with scythes. An important thing to note is that the "Flame Art" typing is not the same as "Fire" typing. There is no fire typing available if you assign this ashe of war. However, the "flame art" makes the weapons scale with str, dex, and faith. This is very interesting and unique because it allows the player to give a weapon faith scaling (that otherwise wouldn't, like the grave scythe or regular scythe) AND also not force the weapon to be "sacred". This allows the player to still imbue their weapon and maintain good faith scaling.

It's kinda meh. The final blast knocks enemies into the air which is nice, but it's pretty slow for what it does overall. One other unfortunate consequence is that using this Ash will wipe any weapon buffs you've applied, so if you've put Blood Grease or Bloodflame Blade or anything like that on your weapon, that's gone after you finish the Tornado.
Would actually be really nice if it left your weapon enchanted with Black Flames for a while after the cast, like Lightning Slash and Flaming Strike do for their respective elements, but it is what it is.