eis

From Proto-Hellenic *en-s (compare the Ionic form ἐνς (ens), later ἐς (es)), from Proto-Indo-European *h₁én (“in”), from which also ἐν (en, “in, at, on”). Possibly was created to contrast with ἐν (en) as the result of analogy with the pair ἐκ (ek) / ἐξ (ex), of which the latter is the older form (see *h₁éǵʰs); the -/s/ in ἐξ (ex) is ultimately from the PIE genitive–ablative marker *-(e)s.

The accusative is from the pre-PIE directional.