If I have Rengar Trophy Hunter in trash and play Cursed Sarcophagus, can I still play the Rengar with ambush on my opponent's turn? Or does it need to be on my turn to be able to play units with Cursed Sarcophagus?

The Ruling

No, you cannot play Rengar, Trophy Hunter using Cursed Sarcophagus on your opponent's turn.

Reasoning

1. Activated Ability Timing: Cursed Sarcophagus features an activated ability. According to Rule 381, all activated abilities can only be activated during the controlling player’s turn and during an Open State. Therefore, you cannot activate the ability of Cursed Sarcophagus on your opponent’s turn.
2. Playing from Trash: Because the ability to play a unit from the banishment zone via Cursed Sarcophagus is restricted to the active player's turn, it cannot be used to bypass timing restrictions on the opponent's turn.
3. Ambush and Permissions: While Rengar, Trophy Hunter has the Ambush keyword (allowing him to be played at reaction speed), the gear Cursed Sarcophagus is the source of the play effect. Because the gear's ability itself is restricted to your own turn, the reaction speed of the unit being played is irrelevant to the timing of the gear's activation.

Important Clarifications

• Triggered Abilities: You are not playing Rengar as a triggered ability; you are playing him as a card from the banishment zone via the activated ability of a gear.
• Ambush Limitations: Rengar must still be played to a valid location. According to the mechanics of Ambush and his own text, he must be played to a battlefield where you control units, or a battlefield containing enemy units. You cannot play him to your base unless another card effect explicitly permits it.

Note: I could not fully verify this ruling against the retrieved rules.