Well I took this puppy on (WAR38/WHM19) and just as he was about to die, he self-detonated, and hit me for 535 HP. I fell like a sack of potatoes off a delivery dock.
I had also some 500+ hp when he took me down. I was 31 lvl war/monk with 167 of DEF with the buffs. Just stay away from that one (unless you're 40+ and very confident) the drop is definitely not worth it.
We had a full and balanced party of 6 Lev-27's at full health and decided to try Fatuus. Everything went just fine until he exploded. He hit me for 384HP and killed 3 of our group outright. Just FYI in case you are around 27-30 and decide to kill it.
Yeah.. it defitnitly is "Ignis Fatuus." Not only do I take latin.. but I JUST saw the mob and recognized the word "Fatuus," being stupid. So I came and checked here. There's no Gatuus to my knowledge.
To be totally random, Ignis Fatuus is latin for foolish flame, and is a poetic way of calling someone a hyperactive idiot. I'm actually blown away by the entirety of the names they've used in this game, ranging from this to the obvious greater mythologies, even down to Indonesian folklore.
... So, yeah. Not all names are necessarily silly.
a little addition to what you've already said here, but i know it's also been used to describe paranormal stuff like "ghost lights". it meaning here, that anyone tryin to follow or catch it is thought to put themselves in harms way. on a side note, ghost lights have also been known as "will-'o-the-wisps". so that kinda makes me think thats where they got the name for these. seein how theyre 2 names for the same thing.