Merlin fidgeted at the question, unsure about how much he should say, how much he wanted to say. He didn't know how he was even supposed to tell Zach that he saw himself arched up, screaming, saw his hands white-knuckled, saw his struggle for breath, could only assume he was in pain because after his experiences with relationships back in the Muggle world why would he want to be in one now? Or ever again.
He didn't even want to bring that up with himself, could never do it with someone else.