Where are the Dark Menhirs during the Grandmaster Battle?

So it looks like it's not a boss? If not, then it should be able to be poisoned. Too bad it's still unkillable due to its disappearing after 30 seconds, which doesn't give poison enough time to work it down to powerguard-able status =[

I think the damage that'll show up per second will be a blue heal-like 12499, like the blood stirges. Probably has something to do with going over the damage cap.