How to get fire incantation

Found a Easter egg unsure if it’s know yet Step one Get the ankh charm from a zombie Step 2 There will be 3 bomb fires around the castle courtyard light them with Molotovs Step 3 there will be a small gold puddle in the court yard interact with it to place the ankh charm it then turns into a soul box Step 4 the box will turn gold melee this with a fire sword (unsure if it has to be the fire sword or any sword the fire was what I used)

I’ll try and attach photos of the bombfire locations unsure how to attach gameplay of the incantation but it’s basically a spell in your tactical slot that ignites all zombies it’s pretty strong