Can I use Hunter’s Mark, Zephyr Strike and Dread Ambusher in the same round?

Question

I have a 3rd level gloom stalker and I have an idea. I have doubts that this will will work according to RAW and I just want to make sure.

A little background. Five highway bandits in a wooded area robbed a friend of the party and I know where to find them or at least a real good idea.

My basic idea is

1. Cast Hunters Mark to target a bandit before combat starts — I haven't read anything saying that this starts combat

2. Start combat and make an attack which benefits from Hunter’s Mark. The rest of the party is going to try to stay hidden

3. Cast Zephyr Strike which ends Hunter’s Mark

4. Attack with Dread Ambusher

5. Run if need be. I know they have ranged weapons and I want to make use of cover

I am a little unclear if I can cast a spell in between attacks. As a side note I am trying a melee based ranger and have the Polearm Master feat.

Answer 1

There are a few potential concerns with this plan. Some of them work out in your favor, others do not.

Bonus action between attacks

You can take bonus actions between attacks, per the answer to this question.

Using both hunter's mark and zephyr strike

You can apply one spell to the first attack, and the second spell to the next attack. Of course, hunter's mark will end, but you'll still have access to the additional damage and advantage from zephyr strike.

Casting hunter's mark without initiating combat

This is the tricky part. "Initiating combat" isn't quite the right phrase here; there's no action you can take that will definitively do so. Whether combat has started is the DM's decision. However, it is possible that they will decide that combat has begun once you cast the spell. This is because the spell has verbal components, and casting it may make some noise.

If you are relying on being hidden to keep you out of combat, casting this spell may give away your position, potentially costing you the element of surprise. However, at a range of 90 ft, it's entirely possible that you can do so without attracting attention.

Benefiting from Dread Ambusher

Exactly when you benefit from Dread Ambusher is a bit tricky in these situations, but you should gain its additional attack at some point. If your targets have particularly good hearing (perhaps they've tamed a creature with the Keen Hearing trait,) your targets might be the ones initiating combat. In this case, you'll still have all the benefits you describe, it's just that your opponents might have already begun moving against you before you use them.

If you remain hidden after casting your first spell, all should go as you describe.

Answer 2

Since Hunter's Mark can be moved from one target to another, and your intent it to have it active for less than one round (on your enemies). You could cast it on a worm (and declare it as your quarry). Then you kill the worm there (or bring it with you while you make your approach. This could take several rounds of movement and stealth, but Hunter's mark persists for up to a minute. As long as less than a minute has gone by before the initial casting, when you're within range you can mark the new creature and begin your attacks as described. This way your initial casting can be done well out of earshot of your eventual target.

Answer 3

Hunter's Mark and Zephyr Strike are both concentration. Concentration effects do not "linger" until the end of the turn, once you've cast Zephyr Strike, your Hunter's Mark will end.