This Soul Eater-looking creature is a protean scourge: one of the few monsters that mostly known in TO-circles for being ridiculously overpowered. Why, you may ask? Stay tuned and find out!
The scourge's chassis is not particularly problematic. Its type being monstrous humanoid rather than outsider is slightly surprising, and more importantly makes its 13 HD a bit less appealing. Stats are all in the 14-20 range, which isn't bad but hardly impressive by this level either. +10 natural armor, a 1d8 gore attack, and two 2d6 claw attacks are welcome, but again, not revolutionary. 40 ft. land speed is nice, 10 ft. climb speed is occasionally useful I guess.
Furthermore, scourges have DR 10/silver, immunity to petrification and poison, resistance to cold and electricity 10, and SR 24. (I am really getting the impression these were initially written up as fiends). Scourges also get Change Shape, allowing them to assume the form of any medium humanoid.
Apart from those traits, protean scourges cast spells as 8th-level sorcerers. That's nice, and means they can reach 7th-level spells by ECL 20. However, it's still not their main selling point.
Split, however, is. Like many oozes, protean scourges can split in two when damaged (by any source, I should emphasize). The resulting new scourge has every ability the original has (including Split), with the only exception being that both scourges divide spell slots among them and both lose Change Shape until they recombine.
The copies don't even divide hit points, meaning that a protean scourge with 110 HP who takes four points of damage splits into two scourges with 106 HP. That's huge: where something like an Elder Black Pudding can only split into around 16 puddings before requiring magical healing, a protean scourge could theoretically (and within a few minutes) turn into 2HP-1 copies.
Spoiler: Numbers get big fast
To elaborate on the formula: a scourge with X hp can turn itself into two scourges that both have (X-1) HP by dealing itself a single point of damage (which is as simple as stepping on a caltrop). Those scourges both turn themselves into two scourges using the same method. In other words,
Also note that since scourges can step on caltrops simultaneously, the entire process only takes a number of rounds equal to the original scourges HP-1, or about 10-30 minutes for the numbers we're dealing with here.
For the default 110 HP scourge, the number of copies that can be created is about 649037107316853453566312041152512. To put this into perspective, you could put a million scourges around every single star in the universe and still have a ton of them left-over.
Now let's assume we have a protean scourge PC who puts a 10 in constitution, takes levels in a d4 HD class, and doesn't take any HP-boosting feats. By level 20, they'd still have 131 HP, which is good for 1361129467683753853853498429727072845824 scourges: enough to give each of the original scourge's copies a million buddies and, again, have left-overs.
However, the real abuse is yet to come. Imagine a scourge who puts an 18 in constitution, gets a +6 item, only takes levels in d12 classes, and puts the two stat increases after level 13 in constitution as well. The end result would be a scourge with 8+12d8+7d12+240 = 347 HP. 2346 is such an insane number that I have trouble properly expressing it, but I'll try.
Imagine densely packing scourges together until you have, say, a cubic foot for each of them. This would already break every contortionist world record (as well as their spines), and yet you wouldn't have enough space. Not in the entire universe, not in a million universes, not in a billion universes. You could assign a scourge (or a billion scourges, or a trillion scourges) to every atom in existence and still have uncountable left over. The protean scourge is a beautiful, insane, almost poetic thing that abandons all logic and reason for the ability to completely fill the universe with itself in a little less than 30 minutes, and I love it.
Also note that all this is before magical healing.
Simply put, the protean scourge's Split is absolutely ridiculous to an extent that borders on the incomprehensible. Imo that's worth an asterisk.
As far as its other abilities go, they're nice but hardly overpowered. Casting is very strong, but being behind five levels (or six, compared to wizards) sucks. In the end I could be happy with either +0* or -0* LA, and will go for the former for now.