I don't see any way to go from level 2 to 3 without flying, there is a massive pit and a rocky tunnel connection. So I don't think a wildfire could have traveled either up or down to everywhere. There is still a lot of vegetation, spider webs, and large trees as well.
So probably a flying fire breather that was able to massacre everything. Are they omniscient and can always find other creatures? The sheer range that I am still finding scores of bodies is surprising. I guess that explains my sudden massive loss of FPS.
I found a named body, but I can't get any information about it other than it is a mangled skeleton.
EDIT:
Apparently that wasn't the only forgotten beast. I think I found the culprit. It's still alive. "A gigantic blob composed of flame. It has a spiral shell and it undulates rhythmically."
It has a blobbed body composed of fire and a shell? I guess that it how it was able to kill such a vast number of creatures and presumably the other Forgotten Beast. Must be super tanky. Is there a way I can view it's kill count with DFHack? It won't let me view it like I can my dwarfs.
EDIT2: Well that was anti climatic due to what I believe is a bugged Forgotten Beast. Apparently it was able to kill everything not only because it could lob fireballs, but it seemed virtually impossible to attack it. I had a full squad of experienced fighters and a near legendary fighter within a few blocks of it. When I ordered them to attack it, they just stood there and did absolutely nothing. I told them to move on a path that would make them cross it, they did so and the only one of them attacked it. There was 4 pages of the Dwarf dodging the blobs attacks, making only 1 attack, a bite which hit. Then the Dwarf died.
I brought my Marksdwarves down and they shot it a single time with a yak bone bolt and it died instantly.
So for whatever reason it was nearly impossible to be targeted by melee attacks even by a group. So even though it seemed to have the equivalent to 1 hit point, it nearly sterilized all levels of the cavern and vaporized my Dwarves and their steel gear and apparently an artifact that someone was carrying.