### Yes, they can cast a spell instead of an opportunity attack when an enemy moves away from their echo

The [War Caster][1] feat (PHB, p. 170) states that you can cast a spell instead of making an opportunity attack "when a hostile creature's movement provokes an opportunity attack from **you**" (emphasis mine).

Initially, I was tempted to say that the opportunity attack is provoked from the Echo, not "you". However, the text for the [Echo Knight][2]'s [Manifest Echo][3] feature (EGtW, p. 183) is quite clear that *you* make the opportunity attack from the echo's position; therefore, the attack is eligible to be replaced by a spell.

However, as illustrated by V2Blast's answer in this [follow-up question][4], the spell originates from your space, not your Echo's.


  [1]: https://www.dndbeyond.com/feats/war-caster
  [2]: https://www.dndbeyond.com/classes/fighter#EchoKnight
  [3]: https://www.dndbeyond.com/classes/fighter#ManifestEcho-1810360
  [4]: https://rpg.stackexchange.com/q/177205/56975