Can I play Rengar bouncing when my opponent is conquering and I have no units there?
To answer your question, no, you cannot play Rengar, Pouncing to a battlefield your opponent is currently conquering if you have no units there.
Reasoning:
- Placement Restrictions: According to his card text and supporting RiftJudge FAQs (specifically FAQ #9634 and FAQ #427), Rengar can only be played to:
- Your base.
- A battlefield you control.
- A battlefield you are currently attacking.
- Definition of Attacking: Simply contesting a battlefield (or defending against an opponent who is in the process of conquering it) does not grant you the "attacker" designation. Because you are not the player who initiated the move to that battlefield, you do not meet the "attacking" criteria required to play Rengar there.
- No Units Present: Since you have no units at that battlefield and do not control it, you have no legal location to place him at that specific battlefield.
Summary: Unless you already control the battlefield or were the player who initiated the attack on it, Rengar's specific play restriction cannot be satisfied. You could, however, play him to your base or to another battlefield you currently control, provided you have the necessary energy.