I personally think it's Onkis, as Kellhus later goes on to lament how despite all his achievements, he's still a slave to the circuit of cause and effect.
H mentioned on a cast recording that it's probably a sorcerous instrument keeping him grounded in reality while exploring the thousand hells.
I believe someone else suggested it might be a reference to your neural system - the head on a pole being your brain and your spine, just behind your face, as in everything you experience. Related to Bakker's Blind Brain Theory.
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