Without being eaten, that octopus returned to the sea.
Literal
Eaten-being [without-ことなく] that octopus [topic-は], sea [to-に] returned [narrative-のであった].
A charmingly literary ending for an octopus story. ~ことなく (without doing) is the bookish alternative to ~ないで or ~ずに. のであった is a narrative past construction — the explanatory の plus the formal であった creates a storybook closure: 'and so it was that...'