Do the Hunter ranger's Horde Breaker and Volley features interact?

Question

The Hunter ranger gets the Hunter's Prey feature at 3rd level, one of the options for which is:

Horde Breaker. Once on each of your turns when you make a weapon attack, you can make another attack with the same weapon against a different creature that is within 5 feet of the original target and within range of your weapon.

One of the options for their 11th-level Multiattack feature is Volley:

Volley. You can use your action to make a ranged attack against any number of creatures within 10 feet of a point you can see within your weapon’s range. You must have ammunition for each target, as normal, and you make a separate attack roll for each target.

Now let's say that a Ranger with a bow uses Volley on a group of three creatures: C1, C2, and C3.

Can he use also Horde Breaker on one creature belonging to this group, e.g. C1, if C2 is considered the original target?

Answer 1

Yes

There is nothing that prohibits this interaction. The ranged attacks made from Volley count as weapon attacks, so any of them can be used as the "trigger" for Horde Breaker.

Note however that Horde Breaker allows you to do this only once on each of your turns, so you can't trigger it on each of the Volley attacks, but only on one.

In your example, you can attack each of C1, C2, and C3 once from Volley, and another one of them a second time, as long as the target of the Horde Breaker attack is within 5 feet of any of the original targets (so if C3 is further away than 5 ft from both C1 and C2, C3 will not be eligible for the Horde Breaker extra attack).