Quote from 1000mAh »What he says: Dominus of Fealtytargets the Defense of the Heart and he immediately gains control of it (before the stack resolves) and thus it would still be his at the beginning of upkeep and he can sacrifice it.
what is the ruling in this case?
DO YOU GET CONTROL OF A CARD WITH DOMINUS OF FEALTYIN A MANNER THAT ALLOWS YOU TO TRIGGER THE TARGETED CARDS ''AT THE BEGINNING OF YOUR UPKEEP'' TRIGGER. SO IS THE CARD IN YOUR CONTROL DURING ''AT THE BEGINNING OF YOUR UPKEEP'' OR JUST ''DURING YOUR UPKEEP''
The "beginning of your upkeep" is just what it says it is. The beginning of the upkeep. Once you are in your upkeep, you have passed the beginning. All things that trigger at the beginning of your upkeep trigger simultaneously. If you control all the triggers, you order how they go on the stack. They resolve in the order you have chosen, with the top item resolving first. If your opponent has any triggers which will trigger at your upkeep (e.g. a card that triggers each upkeep, like Tendershoot Dryad which triggers on each upkeep), then your opponent will put their triggers on top of your triggers, and your opponents will resolve before yours (in the order they have chosen for their triggers, top item resolving first).
Since Dominus of Fealty triggers at the beginning of your upkeep, the resolution of its trigger necessarily occurs after the beginning of your upkeep has passed. Therefore, any permanent you gain control over will not trigger once you gain possession of it if its trigger is at the beginning of your upkeep... because as stated, that has passed. So no, you cannot gain control of Defense of the Heart and then sac it to its trigger, because its trigger did not happen.
To counter what your friend said, it is impossible for him to gain control of Defense of the Heart immediately, without anything from the stack resolving. The very fact of gaining control of a permanent via Dominus of Fealty is the resolution of its trigger on the stack.