Wait, so is that a dark tower civ?
This is confusing, there doesn't seem to be anything unusual about it. Maybe something else is causing the problem for me.
Have you had any joy getting it working yet? I've tried playing around a bit but I'm a modding newbie and can't get it working for my dark tower civ (trying with orcs & giants). Haven't tried for others.
Is [RESPONSIBILITY:LAW_MAKING] definitely necessary for a megabeast to assume the position?
I've gotten it to work with non-dark fortress civs, but still no luck with the dark fortresses. RESPONSIBILITY:LAW_MAKING is the position the beast will take over, but if the civ doesn't have one and you have the VARIABLE_POSITIONS:LAW_MAKING tag instead, the beast will create a lawmaker position on its own.
My guess is that the demon in charge of dark fortresses by default is blocking anyone else from taking the throne. On rare occasions I've seen someone kill the demon in worldgen, but megabeasts taking over is a rare occurance as well, and I've never seen one happen after the other.
I was mistaken before about the demon stepping down from rulership in year 1, it was actually stepping down from 'ruling' its own spire and going to take over the civ instead.