What are the restrictions on the use of Slayer's Prey?

Question

In the recent book Xanathar's Guide to Everything, there are some subclasses/archetypes added for Rangers, among them the Monster Slayer (p. 43), which gives some specific helpful features.

Hunter's Sense

At 3rd level, you gain the ability to peer at a creature and magically discern how best to hurt it. As an action, choose one creature you can see within 60 feet of you. You immediately learn whether the creature has any damage immunities, resistances, or vulnerabilities and what they are. If the creature is hidden from divination magic, you sense that it has no damage immunities, resistances, or vulnerabilities.

You can use this feature a number of times equal to your Wisdom modifier (minimum of once). You regain all expended uses of it when you finish a long rest.

The limitations on use are clearly spelled out.

Slayer's Prey

Starting at 3rd level, you can focus your ire on one foe, increasing the harm you inflict on it. As a bonus action, you designate one creature you can see within 60 feet of you as the target of this feature. The first time each turn that you hit that target with a weapon attack, it takes an extra 1d6 damage from the weapon.

This benefit lasts until you finish a short or long rest. It ends early if you designate a different creature.

So from this, I would interpret it as it being able to switch from creature to creature at will, but there is no guidance as to what happens if the creature under the mark dies, or if you can carry it (so to speak) from encounter to encounter.

Is there a limitation on the use of Slayer's Prey in the wording that I am missing? Or is this a basically a multiple use feature? (i.e., you can designate any creature as a bonus action, with no limitations on uses per day)?

The extra d6 seems a little overpowering at lower levels, and somewhat meh at higher levels.

FWIW, the GM basically allowed it as multi-use for the session, and was going to also try to find clarification.

Answer 1

Restriction 1: It is a bonus action

As a bonus action...

You can only take one bonus action per turn, so you can use this ability at most that often.

Restriction 2: It only works on one creature at a time

...you designate one creature you can see within 60 feet of you as the target of this feature.

...

It ends early if you designate a different creature.

This is saying that you can target one creature at a time. When you target another creature, the effect stops working on the first.

This may seem a bit ambiguous, but it has been backed up by Jeremy Crawford:

Q: Can I get a quick clarification on the Monster Slayer's ability Slayer's Prey? Does the final sentence mean I can use it as many times as I'd like per short or rest, but only on one creature at a time?

A: That's correct.

(emphasis mine)

You can use it as many times per rest as you want

This benefit lasts until you finish a short or long rest.

This means that if you target a creature with the ability but don't kill it or move it to another creature, the ability will still stop working when you take a rest.

With no other text indicating a restriction of any kind, it would be wrong to assume there is one. And, in fact, Jeremy Crawford also confirmed this per the above tweet by confirming that it works "as many times as I'd like per short or rest".

Specific Questions

What happens if the creature under the mark dies?

If the creature dies, the effect ends and you can, if you want on your turn, spend a bonus action to target a different creature like you did with the first creature.

Can you carry it (so to speak) from encounter to encounter?

If you don't kill the creature or otherwise make it ineligible to for Slayer's Prey, then the ability will keep active until you take a short or long rest, in which case it ends.

Answer 2

Pulling from this unofficial tweet from November 2018 by rules designer Jeremy Crawford, the intention of the writing is that you can use the ability as many times as you like, but can only affect a single target at a time. The lack of wording limiting the usage of Slayer's Prey supports the idea that it is an at-will ability.