When the War Caster feat lets my Echo Knight fighter cast a spell instead of the Manifest Echo feature's opportunity attack, where is it cast from?

Question

This question is inspired by the following one: Can an Echo Knight with the War Caster feat use cantrips for opportunity attacks made by their Echo?

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Part of the Echo Knight fighter's Manifest Echo feature states (EGtW, p. 183; emphasis mine):

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.

And the third benefit of the War Caster feat states (PHB, p. 170; emphasis mine):

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.

I am aware that the spell cast using War Caster does not count as an opportunity attack. And this makes me unsure what happens when a creature provokes an opportunity attack from the echo, but the fighter chooses to use War Caster to cast a spell instead.

When an enemy provokes an opportunity attack by moving away from my echo, and I use War Caster to cast a spell instead, where is the spell cast from?

• Is the spell cast from the echo's space, because the opportunity attack would have been made from that space?
• Is the spell cast from the fighter's space, because Manifest Echo only allows opportunity attacks to be made from the echo's space (and not spells)?
• Or is something else the case?

Answer 1

The spell is cast from your own space.

As you point out, the Echo Knight fighter's Manifest Echo feature says, in part (EGtW, p. 183; emphasis mine):

• When you take the Attack action on your turn, any attack you make with that action can originate from your space or the echo’s space. You make this choice for each attack.
• 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.

While the relevant benefit of the War Caster feat says (PHB, p. 170; emphasis mine):

• 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. The spell must have a casting time of 1 action and must target only that creature.

The Manifest Echo feature says you can make an opportunity attack as if you were in the echo's space. However, the War Caster feat says you can cast a spell as a reaction instead of an opportunity attack; once you're no longer making an opportunity attack, the "as if you were in the echo's space" part of the Manifest Echo feature ceases to apply.

Therefore, the spell from War Caster follows the normal rules for spellcasting (other than timing): it's cast by you from your own space.

Answer 2

Thing is, the specific rule of Manifest Echo allows you only to make an opportunity attack. If you cast a spell it is not classifying for the reaction granted by Manifest Echo anymore.

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.

Those are two different specific rules. In the end, the DM decides which one of those specific rules counts.

Specific rules don't always have to allow something, they can also disallow something by stating what the only things are that can be done.