if i use temporal breach on a unit at rockfall path. what happens to the unit?

The unit is banished and stays in the banishment zone — the "play it back" instruction is ignored.

Temporal Breach's first instruction (banish the unit) resolves normally, removing it from the board. But its second instruction — "its owner plays it to the same location" — is impossible, because Rockfall Path says units can't be played there. That instruction is simply ignored (Rule 359.3.e.6), consistent with the standing ruling that any spell/ability instructing you to play a unit to Rockfall Path resolves with that instruction doing nothing (FAQ 6500). The unit does not return to the battlefield.