The UA spell Spirit Shroud has the following effect:
Until the spell ends, any attack you make deals 1d8 extra damage when you hit a creature within 10 feet of you.
The Echo Knight has the following ability (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.
Assuming that a creature triggers an opportunity attack from an Echo created by an Echo Knight Fighter under the effects of Spirit Shroud that is more than 10 feet away from the creature, would Spirit Shroud's bonus damage apply to the attack?
My uncertainty stems mainly from the fact that it's clear that normal attacks made by the Echo would not gain this bonus (if made from more than 10 feet away), as they simply originate from the Echo:
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
The addition of "as if you were in the echo's space" seems to imply that opportunity attacks for Echos work differently, however.
Best Answer
Yes.
You quoted the necessary text: "as if you were in the echo's space". It's not the echo making the attack, but you attacking as if you were in its space. As a further indication, spirit shroud has a range of self, so it would indeed benefit the opportunity attack you make as if you were in the echo's space.
For a contrasting effect, let's say you were within the aura created by the crusader's mantle spell cast by one of your allies and your echo is not. In this case, the opportunity attack made from the echo's space would not gain the benefits of this spell because you would not be within that aura since you are making it from the echo's space, even though "your real you" still is within it.
You can sort of see it as a quick and temporary teleportation effect.