How does eldritch blast interact with being hidden?

Question

A level 5 goblin warlock moves behind a large tree and uses their bonus action to Hide. Their Stealth check beats the enemy's passive Perception. The hidden warlock then casts eldritch blast and rolls...

1. Both beams with advantage?
2. The first beam with advantage and the second beam without?
3. Neither beam with advantage?

In case it makes a difference, assume that I (as the DM) rule that the enemy is close enough to clearly hear the "chanting of mystic words [...] with specific pitch and resonance" from the verbal components of eldritch blast.

Answer 1

So there are two issues to consider, and my original answer only considered one of these. I can't find a definitive answer, but interpreting RAW to answer:

Issue One: Does a Verbal Component Spoil the Hidden Advantage?

Short Answer: GM's Call

There's no general rule I can find which dictates how loud/obvious verbal components are. There's a couple of specific spells, but nothing general. So it's a GM's call as to whether the verbal component can be heard and understood to be an attack. I'd err on the side of the player in most cases, esp if combat is already underway; the noise of combat would probably cover most verbal components.

If the GM rules the noise reveals the warlock, then none of the attacks will have advantage.

Do Separate Attack Rolls from Eldritch Blast All Use Advantage?

Again, there's no RAW which definitively answers this. But we can infer an answer using two other rules:

Attacking while Hidden, PHB, p195

When a creature can't see you, you have advantage on attack rolls against it. If you are hidden-both unseen and unheard-when you make an attack, you give away your location when the attack hits or misses.

Eldritch Blast Cantrip, PHB, p237

You can direct the beams at the same target or at different ones. Make a separate attack roll for each beam.

Since you're making a separate attack roll, you give away your location on the first roll, so you'd have advantage on the first, but not on the subsequent rolls. There's no wording to suggest the two (or more) blasts are simultaneous.

As a GM, I can imagine a circumstance where a player might convince me otherwise. But that would probably be outside of RAW.

So, in the specific case of your Goblin Warlock, It's GMs call if the first attack gets Advantage, but the second should definitely not.

A lot of D&D is the GM making calls like these. The rules are pretty good as a baseline, but far from comprehensive.

Answer 2

Both beams with advantage

The duration of Eldritch Blast is “Instantaneous” meaning all beams attack at the same time. At that time the warlock is hidden so the attacks are made with advantage.

Indeed, after the warlock casts Erdrich Blast, they might still be hidden. Casting a spell or making an attack does not, of itself, end the hidden “condition” - hidden is not invisibility and has different end states. Neither does making a noise.

It reveals the warlock’s location, but we knew that anyway (“behind a large tree”). Hiding ends when you are seen and only for those creatures that have seen you.

Now, it’s perfectly sensible for a DM to rule that the “circumstances are [no longer] appropriate for hiding” once the warlock has cast the spell but I can see cases where they might be. For example, Eldritch Blast has a range of 120 feet, well beyond the range of most Darkvision or light sources. A goblin sniping with Eldritch Blast from where no one can see them might still be hidden after doing so.

As for losing the hidden state during the casting of a spell: don’t. Within the mechanics of D&D spells with a casting time of 1 (bonus) action are indivisible in time (Counterspell excepted). That is, within the structure of the game, no time passes between starting to cast and finishing casting.