Extract ability against immortality

[Stayingpower]

Hi,

I have a question regarding the extract ability of Mind Flayers.

I have a player in my game that is an Highlander Immortal (unnofficial template) which gives, guess what, immortality. The only way to kill them is by chopping their head off.

I'm wondering how you would rule such an immortal getting his brain extracted by a mind flayer? Would the brain tissue grows back with the slow regeneration ability, resurrecting him? Does an extracted brain is kind of the same as a beheading?
Both interpretations are legit I guess, so I wanted your thoughts on that.

Thanks!

[Malimar]

Not having seen Highlander, I can't speak to that particular canon, but I generally treat having your brain removed as being functionally equivalent to beheading.

[Necroticplague]

The reason beheading something is lethal is because it separates the brain ,which you use to control your body, from the rest of the body. Other than that, there isn't anything intrinsically life-requiring in the head (useful sense organs, yes, but nothing strictly vital). Extracting the brain is actually a step closer to the reason decapitation is lethal. So I would just count decraniation as decapitation for these sorts of purposes.

[Stayingpower]

That's what I had in mind.

I thought of the second option (recovering brain tissue) by thinking of extremely fast healing characters could "react" to being extracted ,like Wolverine or Deadpool.

Thanks!