The third benefit of the Sentinel feat reads (PHB, p. 169-170):
When a creature within 5 feet of you makes an attack against a target
other than you (and that target doesn't have this feat), you can use
your reaction to make a melee weapon attack against the attacking
creature.
In the following scenario, would my Echo Knight fighter's echo get a reaction attack on the enemy from the Sentinel feat's third benefit?
- I can see the echo, an enemy, and my ally
- The echo is within 5 feet of that enemy
- That enemy attacks my ally
Best Answer
Not unless the attacking enemy is within 5 feet of you.
The third benefit of the Sentinel feat reads (PHB, p. 169-170):
The description of the Echo Knight fighter's Manifest Echo feature says (EGtW, p. 183):
The second bullet point of the Manifest Echo feature lets the Echo Knight attack from their echo's space when they take the Attack action. The third benefit lets them make an opportunity attack from the echo's space when a creature moves away from it.
However, neither Manifest Echo nor any other Echo Knight feature lets you make attacks from the echo's space in any other situation, as written. No feature lets your echo count as "you" for the purposes of other reactions that are based on a trigger occurring within a certain distance of you - so you can only take such a reaction if that trigger occurs within that distance of you (not just your echo).
As a result, the reaction from the third benefit of the Sentinel feat won't be triggered by an enemy within 5 feet of your echo who's attacking one of your allies - unless the attacking enemy is within 5 feet of you as well.