Scylla and Charybdis

Scylla and Charybdis must block every turn
if able
The twin monsters that guarded the Strait of Messina, one was a whirlpool, sucking in any ships that tried to pass it, while Scylla's monster heads picked off ships and the sailors on them. Neither was safe to pass, hence the phrase,
"stuck between a rock and a hard place."