Rengar

When you play a unit, give a unit +1 this turn.

[Ambush] (You may play me as a [Reaction] to a battlefield where you have units.)
I can be played to a battlefield where there are enemy units (even if you don't have units there).

[Reaction] (Play any time, even before spells and abilities resolve, including to a battlefield you control.)
[Assault 2] (+2 while I'm an attacker.)
I can be played to a battlefield you're attacking.

[Accelerate] (You may pay 1 as an additional cost to have me enter ready.)
[Assault 2] (+2 while I'm an attacker.)
[Deflect] (Opponents must pay to choose me with a spell or ability.)
[Ganking] (I can move from battlefield to battlefield.)

[Reaction] (Play any time, even before spells and abilities resolve.)
Banish a friendly unit, then its owner plays it to any battlefield, ignoring its cost.