It can gain counters each turn at least one faerie deals combat damage, and that can be an easy thing with all your flying faeries
A door is one half of the split card , and unlocking a door is paying its mana cost to enable its abilities
Then, all of your opponents, not just the one you countered a spell, cannot cast noncreature spells for the rest of the turn
If Yahenni and a creature an opponent controls die simultaneously (perhaps because they fought or were in combat together), Yahenni won't be on the battlefield as its triggered ability resolves
[19] Let's say that Anathemancer is attacking and Arboreal Grazer is blocking
For extra fun, play Reaper King as your commander and overload on Conspiracy and Arcane Adaptation effects to make sure all of your Wizards are also Scarecrows for maximum carnage, allowing you to blow up two permanents with every Wizard you cast once you get Reaper King on the battlefield