I thought that the OL couldn't gain threat when he was attacking with a hero with dark charm, but then lordshinjo showed me this entry in the faq:
Q: Under what circumstances can the overlord receive
threat for rolling surges on an attack roll?
A: The overlord may spend two surges on each attack roll
to gain one threat. He may do this on any attack that hits a
hero. This represents a change from previous FAQ rulings
on this subject.
So, it seems sure that he gains threat through a hero attack. But... how was the rule before this? Why this change? Dark charm is overpowered, but now I can regain the threat spent with the card with a good attack!