If I take control of an opponent's Deathknell creature (like Ferrous Forerunner) using Hostile Takeover or Possession, then kill it (like with Cull the Weak), do I get the Deathknell trigger before it goes back to the opponent's trash?

Ruling: Yes, you get the Deathknell trigger. Deathknell triggers capture all information about the creature right before it's killed, including its current controller.

Nuances:

• Even though the creature returns to the opponent's trash after dying, the controller at the moment of death determines who gets the Deathknell trigger
• In the case of Ferrous Forerunner, the player who controlled it when it died would get the 2 3-might units, not the original owner