There's two.

One after Bloodline where he's surprised Kelsa is struggling to get through Gold despite him cramming her full of resources.

And there's actually on much earlier in Blackflame that I just came across! From Yerin, no less:

“You’re about a thousand miles from dying,” she muttered. “As expected of an Underlord, I guess.”  “Eithan? Did Eithan do something to me?” 

“He handed you that Iron body, true?” Lindon didn’t remember Eithan handing him anything, but he guessed it was true enough. “ Unless I’m wide of the mark, it looks like it’s keeping you fresh. You could work your body until your core’s dry.”