A Quote from the card of Dar Hilzernod, the Great Wyrm's minion:

"When Dar Hilzernod is reinforcing, <something happens>"

Here's the problem: Dar Hilzernod had no minions. Therefore, he cannot spend threat for reinforcements. That being said, his special ability is useless, is it not? If not, and it was intended to be at the beginning of every turn, why mention reinforcement at all?

Reinforcing here does not refer to bringing in new monsters in an encounter, it means "reinforcing a dungeon". This happens when a lieutenant is on the same location as or one trail away from the heroes.

EDIT: Checked the rulebook: It should really read "fortifying" instead of "reinforcing".

Thanks! I totally overlooked that!