Oh this brings me back, i was actually the one who requested this particular death with the green acid sludge, almost a decade ago. Sadly i changed computers twice since then and have long since lost the gifs too, i truly hope someone saved them all and can reshare them.
Besides the first gif i remember the one where the victim fells into pit into all manners of spikes and grinders was a great one too for my tastes.
Btw they weren't made using mugen at all, at least not most of it. I remember the author trying to explain the process, it was actually "relatively" simple but i still couldn't figure it out.
From what i remember, the author had taken gifs of the fatalities/death pits of eternal champions from some site, extracted the characters out somehow (this is the part that confused me) and then added his own characters whose sprites he dled from a spirte ressource site and who were then edited and retouched as appropriate for the fatality.>>10128
They are a gurochan original! Basically a budding animator made a thread using eternal champions gifs to make new fatalities and took requests.
Really hope these gems can be shared again!