Hi all,

  I am DM'ing a game where I have a party with a Gloomstalker Ranger.  We are currently in a cave dungeon where there are no light sources and all creatures have darkvision. 
The Gloomstalker has Umbral Vision making him invisible to all creatures in the dungeon.  He also casts Pass Without a Trace spell negating movement based sounds/tracks.

An issue came up with combat: 
Gloomstalker is attacking monsters in direct line of sight of other monsters, then moving position without breaking line of sight (no Hide action either).  As the DM, i rule that the monsters can see the attacks happening and have a general idea of where he ends up and can make attacks against him at disadvantage (attacking an invisible creature).  He argues the monsters shouldn't be able to attack him at all, as he is invisible and can't be heard moving.

Thoughts?  Did I make the right call?  Or is this Gloomstalker build simply not target-able in this kind of situation.  It's a lower level dungeon, so options on what monsters can do are very limited.

I am a newer DM.   Thanks for any advice

The invisible condition does not make one unattackable, it only imposes disadvantage. As you said, nothing is preventing the monsters from seeing other monsters getting attacked by something. And Umbral Sight only affects creatures relying on darkvision to see him, so if you have any creatures with blindsight, those would be able to perceive it. I could see an argument for tremor sense counting as 'tracking' enough for Pass Without Trace to prevent it working, but frankly I'd let tremor sense work also. There are also a few CR2 creatures with truesight, which would also overcome Umbral Sight.

His argument is that he moved after he attacked, so the monsters would go to where he made the attack from, not to his location.

I made the argument that invisibility is not perfect.  whether it is camouflage or magical, if a monster is aware of your presence, they can see your movement if not you directly.   (think a man in a ghillie suit, or the Predator's transparent cloaking moving through the woods..  you cant really see them, but you can see their movement and where they are moving).

I guess what im asking is:
should i let him have his way (to effectively be un-targetable in these situations)?
or
Is my reasoning sound and still make him attackable (but with disadvantage because he still invisible)?

i am only referencing encounters containing monstsers with darkvision  (skeletons, zombies, bugbears, etc)

In 5e combat, unless a creature has taken the Hide Action, their position is known to all other creatures on the field. That is why invisibility makes enemies attack at disadvantage and many spells that target a specific creature rather than AoE require the caster to see the target. Invisibility alone does not make a creature undetectable, and Pass Without Trace does not mute sounds, so you can say the monsters have a rough idea of his position based on the sound of his movements if he has not taken the Hide Action.

Quote from The_Ace_of_Rogues >>

In 5e combat, unless a creature has taken the Hide Action, their position is known to all other creatures on the field. That is why invisibility makes enemies attack at disadvantage and many spells that target a specific creature rather than AoE require the caster to see the target. Invisibility alone does not make a creature undetectable, and Pass Without Trace does not mute sounds, so you can say the monsters have a rough idea of his position based on the sound of his movements if he has not taken the Hide Action.

Agreed. The "veil of shadows and silence" mentioned in Pass Without Trace's description is just flavor text; it doesn't literally make your steps silent. The +10 it grants to Stealth checks doesn't apply in combat if a character isn't hiding via the Hide action.

If you want an in-universe way to explain this to your player, remind him that even if his steps were totally silent, he'd still need to breathe. Unless he's taking time to steady his breathing (as part of a Hide action, for example) the monsters can probably hear him as he works up a sweat in combat.

For future encounters in the cave, you might want to look into including a few creatures with Blindsight or Tremorsense; either will bypass Umbral Sight. Pseudodragons, all kinds of Wyrmlings, Flail Snails and Water Weirds all have Blindsight, and Young Kruthiks have Tremorsense (as do the adults).

Important: Don't make every creature they encounter for the rest of the adventure have Blindsight; that's just petty. But you can include a handful so your Ranger has to work at least a little bit.

The only way the ranger could be completely silent would be if he was in the area of a Silence spell or something similar. Foot steps by sound or appearance, the rustle of cloth, the smell of oiled equipment, the sound of armor, the passage of air - all things that can give away position even if you cannot be seen. 

The ranger needs to take up the Hide action if they don't want to be attacked and even then the enemy can make Perception checks against his Stealth roll to see if they detect him.

The way invisibility works is that unless you use the Hide Action, they know where you are. You're not fully distinct, do attacks against you are at Disadvantage, but they know where you are.

If you take the Hide Action (which can be a Bonus Action with certain classes like the Rogue and so forth, but generally, it's an Action), they don't know where you are, if they don't pass a Wisdom (Perception) check that's higher than your Dexterity (Stealth) roll. That means they have to guess where you are as well as strike at Disadvantage.

Pass Without Trace simply adds 10 to your Dexterity (Stealth) check. That's it.

Now, that's how the system works. Everything else is just trying to make that intuitive for players to understand, but that's how it works.

If your Gloomstalker Ranger hasn't taken the Hide Action, then the monsters know where he is, but attack with Disadvantage.

You have the right of it; you can attack invisible creatures but with disadvantage. 

Also I'm just going to give you the best advice I can WRT to DMing: you're job is to make rulings, not enforce the rules. If you make a call that isn't in line with what the rules expressley say then what you say goes and as long as it's both fair and internally consistent then there isn't a real problem.

Also I'm going to assume your Gloomstalker is a new player as well, since that's about the only way I can see someone taking an incredibly powerful ability and trying to make it utterly broken.

This is also a good time to remind your entire table of players that the D&D rules set is just that. A set of rules for a game. They are not an attempt at being a reality simulator. This means finding a logical trap that creates situations like this isn't relevant. What the rules say is.

Gloomstalkers in this environment already get to shine. I love that! They don't need more to be above and beyond what other characters are doing in this situation.

The best evidence that Invisible is not Hidden is just the sheer cost of the Hide action. Most characters basically can't do anything else on their turn if they want to hide. It is intentionally difficult to do.

Also, pass without trace does not render you completely silent and undetectable in combat. "Tracking" in D&D parlance is following a creature's trail outside of combat, not keeping track of where a creature is during combat. If it masked all sounds (and smells), it would say that. There is an argument that being invisible within an area of silence could make you untraceable to some enemies, but enemy detection can also involve senses like smell and potentially other supernatural senses like being able to detect magic or your ethereal displacement or literally anything. This is not covered well in the rules, but it's generally thought that if silent + invisible = hidden, it would be in writing somewhere.

I think you have the right direction on Gloomstalker mechanics. Attacks against other monsters doesn't mean they don't notice their friends dying they just have trouble fighting him, the same way players have trouble fighting an umber hulk, but monsters don't have to be stupid. Darkvision is helpful, but even creatures with darkvision use torches or other sources of fire or light to see their environment better. Don't negate his abilities entirely, but it is fair to level the playing field by adding light!

You made the right call. 

Pass Without Trace negates tracks. It does not negate sound, at most it muffles it. And it only comes into effect when you're actively Stealthing, where it grants a +10 to the roll. If you're not Stealthing, Pass Without Trace is just preventing you from leaving tracks. If the Ranger is attacking, and hasn't dual classed into Rogue for Cunning Action: Hide, then the Ranger is not stealthing when moving. Yes, he is invisible to creatures using darkvision to look for him, he does not leave tracks but they can still hear him.