The Echo Knight fighter has the Manifest Echo feature, which grants the following ability, among others (EGtW, p. 183):
When a creature that you can see within 5 feet of your echo moves at least 5 feet away from it, you can use your reaction to make an opportunity attack against that creature as if you were in the echo's space.
The War Caster feat includes the following benefit (PHB, p. 170):
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack.
If an Echo Knight has the War Caster feat and knows a spell like, say, booming blade, can they use the spell in place of an opportunity attack provoked by an enemy moving away from the echo?
Best Answer
Yes, they can cast a spell instead of an opportunity attack when an enemy moves away from their echo
The War Caster 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's Manifest Echo 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, the spell originates from your space, not your Echo's.