Willbreaker
"Master your mind, or I shall master it for you."
Artist: Dan Murayama Scott

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Willbreaker
"Master your mind, or I shall master it for you."
Artist: Dan Murayama Scott
Willbreaker
"Master your mind, or I shall master it for you."
Artist: Dan Scott
Willbreaker
"Master your mind, or I shall master it for you."
Artist: Dan Scott
I just had a match in Magic Duels where I used Willbreaker to steal not only my opponent's Westvale Cult Leader, but his BFZ Gideon too. It was glorious.
If I control Willbreaker and I target another players creature with Rubinia Soulsinger, I would gain control of the creature because of Willbreakers ability. If I then untap Rubinia at my upkeep, would I lose control of the creature?
No.
Until Rubinia untaps, you have two effects giving you control of the creature: Rubinia’s ability and Willbreaker’s ability. When Rubinia’s ability ends, Willbreaker’s ability is still giving you control of the creature.
If I have 2 willbreakers on board and target an opponent's creature, so both have to be destroyed for them to get the creature back or is it controller by one of the two?
Both Willbreakers create a control-changing effect giving you control of the creature. If one of those effects ends, the other one will still be granting you control of the creature.
If I've got Willbreaker and Erratic Portal in play, who chose to pay the tax if I activate the portal on an opponent's creature?
Willbreaker’s ability will trigger when you activate Erratic Portal and the triggered ability will resolve first, giving you control of the creature. Then since you control the creature when Erratic Portal’s ability resolves you have to pay 1 to keep your new creature or to send it back to its owner’s hand.
I have a Willbreaker under my control and my opponent has Gideon, Ally of Zendikar. They use Gideon's +1 and I use an instant on Gideon while he's a creature, thus gaining control of him. At the end of turn, when Gideon ceases to be a creature, do I still have control of him or does he returns to my opponent's control? And, whatever the answer, does it apply the same way to manlands?
You keep control of Gideon or the creature land or whatever temporary creature you managed to steal for as long as you control Willbreaker. It doesn’t matter that they stop being creatures when the turn ends, Will only cares that they’re creatures when the trigger goes on the stack and when it resolves.