Continuity mistake: At the beginning of the movie, policemen are inspecting the deceased Rory's house to retrieve the bloody mattress for Dr. Channard yet at the end of the first Hellraiser, the house completely collapsed.
Continuity mistake: In the scene where Channard kills the original Cenobites, the collar of Pinhead's outfit undergoes visible changes. First it leaves his throat exposed but covers the rest of his neck. After Channard turns him back into a human, the collar then covers his entire neck. Finally, the collar completely disappears a second before Channard slashes Pinhead's throat.
Continuity mistake: After Julia punches her arm into Franks' body and grabs his heart out from his skinless body, in the wideshot after she has grabbed his heart out her face is clean, but in the next closeup there is suddenly lots of blood on her chin.
Continuity mistake: When Browning is cutting himself on the mattress, in one shot his right shoulder is covered in blood, but then in the next close up of him, the blood has gone. (00:29:35)
Continuity mistake: After Tiffany solves the box and it closes itself, all the doors to Hell should have closed up again too as the closing of the box means the closing of the doorways; it happened in the first Hellraiser when Frank died the first time and Pinhead closed the box so they could return to Hell.
Continuity mistake: When Frank came back in the first film, he was little more than goo on bones but when Julia comes back she is much more whole. True, she had a lot more blood to use than Frank to begin with, but it still took Frank three bodies' worth of blood to look the way she does when she comes back.
Continuity mistake: When Julia shuts Kyle into the room full of hanging bodies, she begins to kiss Kyle, then starts to suck out his blood. After she sucks all his blood out, between shots the amount and position of blood over her lips and chin, changes. (00:46:55)
Continuity mistake: When Julia emerges from the mattress and kills Browning, the mattress becomes completely saturated with blood. Then when the mover comes to take it away at the very end, there are patches where the mattress is clean. Then again, when he is pulled into it and his friend comes into the room, it is saturated again.
Continuity mistake: When Browning lies down while cutting himself, the mattress under him is dry and his blood is only smeared over his chest. Yet when the shot cuts to Channard and back, the mattress has suddenly become utterly soaked in Browning's blood. (00:30:00)
Continuity mistake: When Dr. Channard is being turned into a Cenobite, his tie disappears in some shots. (01:04:45)
Continuity mistake: When Tiffany goes back to retrieve the diamond shaped box from Julia's grasp, the position of Julia's fingers around the box change between shots, as Tiffany reaches down to grab it out of her hand.
Answer: All we can see (in abstract closeup) is serpent-like tendrils cutting him and driving the pins into his head. As someone else said, it's likely another cenobite, although alternately it could also be the same "contraption" that turns Channard into a cenobite late in the film, given the tendrils are similar to the ones he sprouts.