Precursor Killers: While not actually known with any certainty, it is generally believed that the Precursors were wiped out by… the Precursors. (Being the lovely near-solipsists described in the previous trope, well, once they started interfering with each other’s whims, they just couldn’t cope with each other’s’ existence. Splat. There are other theories, but this is the leading one.)
…so far as the best-known, usually-referred-to-as Precursors go, anyway. Why there is a general lack of extant Precursors around is a whole other problem. Suggestions on a postcard, please, addressed to the Existential Threats PWG, codeword BERSERKER VOID.