Hearthpwn

it's the same as if Unseen Saboteur or Yogg-Saron, Hope's End casts spells with overload.

they don't proc because the Drustvar horror casts the spell and not you.
ooverload is part of the spell's effect so it's applied even when cast by another card's effect, Unbound Elemental and Thunderhead clearly state: "when you play a card with overload", not "when you overload", that's the key difference