namely, what the hell was up with the Channard cenobite? Why the tentacle? Why that particular death?
I will say that the guy playing Channard absolutely wiped the floor with everyone else in this movie, acting-wise. Kristy is solid as always, but he's on a whole other level.
The tentacle gives him more power because it connects him directly to Leviathan. I think that connection is also the reason the sky over the Channard Institute turns into Hell's sky, with Leviathan appearing during flashes of lightning. It's like the connection is allowing more of Hell to come through the gates.
The tentacle began to retract because Leviathan was about to transform into a box, I believe, and since Channard's tentacle blades were stuck in the floor..