So I was playing a game the other day with an older faerie card. I don’t know the exact name of the card but the text says “power and toughness are each equal to the amount of blue permanents you control”. Someone played Bond of Passion, text says “gain control of target permanent until end of turn. Untap that creature. It gains haste until end of turn. Bond of passions deals 2 damage to any other target.”. If the person I was playing with gained control of my faerie and had no blue permanents, would it’s power and toughness stay at what it was when I was controlling it or would it diminish down to zero, equal to his blue permanents, since he is now in control of it until end of turn?
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-MadMage
Faerie Swarm continually checks the number of blue permanents its controller ("you") controls, and adjusts its power and toughness accordingly (C.R. 109.5, 613.1g, 613.4a, 613.5, 604.3). Thus, if your opponent controls Faerie Swarm, it checks how many blue permanents your opponent controls.
Note, however, that normally, Faerie Swarm itself is blue, so it normally counts itself among the blue permanents its controller controls if it's on the battlefield, so that in normal cases, Faerie Swarm's power and toughness on the battlefield will each be at least 1.
EDIT (Nov. 10, 2020): Edited.
EDIT (Dec. 22, 2020): Some rules were renumbered since this post started.
Since they control the Swarm the "you" in the text now refers to them. So, if they control no other blue permanents, its P/T will be 1/1 for the time they control it since they now control 1 blue permanent.
Using card tags is mandatory in the Magic Rulings forum. Please check your private messages to learn how to use them. I've added them for this post.
-MadMage
Note, however, that normally, Faerie Swarm itself is blue, so it normally counts itself among the blue permanents its controller controls if it's on the battlefield, so that in normal cases, Faerie Swarm's power and toughness on the battlefield will each be at least 1.
EDIT (Nov. 10, 2020): Edited.
EDIT (Dec. 22, 2020): Some rules were renumbered since this post started.
Bond of Passion
Faerie Swarm
Since they control the Swarm the "you" in the text now refers to them. So, if they control no other blue permanents, its P/T will be 1/1 for the time they control it since they now control 1 blue permanent.