There's a moment, just a moment, when he hesitates. Just as he steps, arms wide, across the breach into the spirit world. He feels it as he begins to transfer his weight forward, a little bit of pressure in the ball of his foot that's going the wrong way. But his body is already moving forwards, into the darkness. He hears Merlin cry out behind him. He knows that this is the right thing to do – he can't let Merlin die. But even as he is dying for him a small part of him wishes that he was dying by him. Together, even if that's wrong, because at least then no one would be left behind. He doesn't want to hurt Merlin, but this is the better way. He's still not sure which one of them is leaving, which one is being left.
The screaming surrounds him, a chilling cacophony, but he thinks he can still distinguish Merlin's voice through the rest. And for a tiny, tiny fraction of a moment he feels regret. He wishes that they could have had a little more time together; the knight and the warlock. And in that same, tiny fraction of a moment, he turns and sees Merlin as if through a thick grey veil, looking back at him. His lips part, forming the first syllable of Merlin's name. He doesn't know what he intends – a reassurance, a statement, a goodbye. But his breath splinters into a thousand cold shards, sucked into the darkness. The gate between the worlds slams shut. He is gone.