εἰς

Etymology
From (compare the Ionic form, later ), from , from which also. Possibly was created to contrast with as the result of analogy with the pair  /, of which the latter is the older form (see ); the -/s/ in  is ultimately from the PIE genitive–ablative marker.

The accusative is from the pre-PIE directional.

Preposition

 * 1) into