How do you do it.
Ok, specifically, how do you run this thing? Because I did it tonight and in retrospect I'm not sure if I followed the rules for it. The big question is how do you attack it. Does one have to locate and reach the core or can you attack anywhere it manifests eyes or jaws? Because DC 41 is no joke to find, especially if your a couple levels below it.
I cut my players some slack on finding it with some circumstance bonuses, because they needed to roll really high to spot it and that was assuming they Seeked in the right area.
The encounter did feel suitably horrifying in a Fullmetal Alchemist sort of way and they got very lucky with a couple spirit blasts that sent the thing running before it could lock them all down. But if I hadn't given them those bonuses they might never have found it.
Looks very much like you can only attack its "core", and you have to make the DC41 perception check to seek it out. Very nasty monster.
Its weakness is that its fairly slow and is immobilized while implanted, so PCs can generally escape from it and devise an approach to deal with it (assuming they made their recall knowledge after the ground itself started biting them).