So just out of curiosity, as this came up recently in one of the games I'm in, where we run into a pack of ... hell hounds(?), but what are some anti-invisibility measures for your average joe, meaning tactics that don't require a lot of preparation time or the usage of spells that are hidden behind a level lock or outright impossible to use for certain classes? I mean, I know there seem to be some crowd favorites, like the good ol bag of flour for example, as risky as that can be, and I recently had one of my characters use some ink from his travel diary on his weapon in order to try and mark the enemies as he slashed at them, but are there any more you guys are aware off? Interestingly enough, I once saw someone suggest a flask of beer as an alternative, though I don't quite understand how that would work, but yeah. Thanks for your time guys.

Faerie Fire is the usual go-to and is highly recommended someone take it in all parties. I'm not aware that Hell Hounds go invisible but depending on the ability Dispel Magic might work. 

Is it a Shadow Hound?  The Shadow Hound ability requires concentration so usual things to interrupt a caster would work like area of effects, etc. It loses this ability when it attacks so you could try to absorb the first hit then use your turns tearing up before it uses the ability again.

Note if you know where it is located (runs, stops, uses ability, turn ends), you only get disadvantage on the attack roll. Get advantage through literally every other means and they cancel out.

Once you know where it is located, I recommend ganging up on it. Shadow Hounds (again assuming thats what it is) have low HP and low AC relative other creatures. A good hit from a strong fighter could one-shot it.

You can grapple the invisible creature to keep it from moving. A pool of water or mud on the ground can give away a location too. Aoe attacks only need to know the general location of enemies. You can also use an action to look for signs of the invisible enemy and point out its location for allies. Lastly, the alert and observant feats can reduce the disadvantage of being ambushed by invisible enemies.

A bag of ashes or powdered charcoal can serve the same function as a bag of flour, but without the risk of explosion. Ashes are also free.

Oil poured on the floor and then lit is a good way to provide some area control. Your invisible opponent is likely to avoid the flaming oil. You can use this to channel its movement.

If you have the cash, caster and time to spare, make a Lantern of Revealing.

Ball Bearings and Caltrops

Create Water works indoors or someplace where you can reliably cover the floor with water. Unless they are invisible and also flying. 

Ever played Shadowrun? Cans of paint, or even some variety of paint grenade, work wonders against invisible beings. (Unless your DM rules that the paint immediately turns invisible. Depends on how your DM rewards creativity. 

Keep in mind that being invisible doesn't mean you can't be detected by mundane means. If you are invisible and you do something that would give away your location (such as attacking), even if you keep your invisibility, enemies can track your movement until you take the Hide action. Invisibility itself only makes you heavily obscured if the Hide action isn't also used. Invisibility still makes it harder to hit you, since enemies have disadvantage to hit and you have advantage to hit. In addition, once you are fully hidden from view, enemies have to guess where you are, and if they guess wrong, their attacks miss.

I'm a little annoyed that the rules for invisibility aren't all in one nice convenient spot. My party was almost TPKed, with me and one other person the sole survivors, and we probably could have done better had we known that we should still have been able to know where the enemy was.

Quote from dluxcru >>

Ever played Shadowrun? Cans of paint, or even some variety of paint grenade, work wonders against invisible beings. (Unless your DM rules that the paint immediately turns invisible. Depends on how your DM rewards creativity. 

FWIW, Crawford has Saged Adviced that invisibility only covers what you are wearing/holding at the time of casting. I mean, it's still up to the DM as you say. 

And, honestly, as a DM I'm likely to leave different kinds of invisibility open to working differently. I see no reason why a creature that can turn invisible naturally (Hell Hound, Imp, etc) would have to have the very same kind of invisibility that the spell would create. For ease of game rules, sure. But a Hell Hound that is visible for a round after you throw the paint and then invisible again after seems fine by me :)

Quote from Thucydides >>

 

had we known that we should still have been able to know where the enemy was.

There was another thread about this recently, but it's not entirely clear whether you get to know that. The text for unseen attackers and targets is somewhat poorly worded. There's an interpretation where you still have to guess where the opponent is, and might guess wrong and therefore auto miss, even if they aren't Hiding. Another interpretation is that you do still know where they are automatically. But imo it's not clear.

A while back, the party in my campaign was fighting a coven of green hags in a swamp. The rogue attempted to use the bag of flour to detect the invisible hag, but since everything the invisible hag is carrying turns invisible too, I just had the bag of flour momentarily indicate where the hag was so that he could make an attack (with disadvantage) against that square. Seemed like a reasonable compromise.

Quote from TexasDevin >>

A while back, the party in my campaign was fighting a coven of green hags in a swamp. The rogue attempted to use the bag of flour to detect the invisible hag, but since everything the invisible hag is carrying turns invisible too, I just had the bag of flour momentarily indicate where the hag was so that he could make an attack (with disadvantage) against that square. Seemed like a reasonable compromise.

See I think that's reasonable, even if it's anti-Crawford. Is that a hag specific rule, or a general invisibility house rule? :)

In the case of the green hag, they can go invisible as an action. Not just that, but it's a super powerful version of invisibility.

Invisible Passage. The hag magically turns invisible until she attacks or casts a spell, or until her concentration ends (as if concentrating on a spell). While invisible, she leaves no physical evidence of her passage, so she can be tracked only by magic. Any equipment she wears or carries is invisible with her.

Quote from TexasDevin >>

In the case of the green hag, they can go invisible as an action. Not just that, but it's a super powerful version of invisibility.

Invisible Passage. The hag magically turns invisible until she attacks or casts a spell, or until her concentration ends (as if concentrating on a spell). While invisible, she leaves no physical evidence of her passage, so she can be tracked only by magic. Any equipment she wears or carries is invisible with her.

I wasn't aware of the 'no evidence' part, that's cool.

The 'any thing she carries/wears' part is something Crawford has weighed in on, that was one reason I asked. I'm one to question RAW all the time, but just to point out. :) The Sage Advice clarification is that for the invisibility spell at least, the stuff carried or worn when the spell is cast is the stuff that becomes invisible. Things you pick up or put on later would still be visible. Although Crawford did say that he'd allow someone invisible to do something like tuck an item into their invisible pocket and make it invisible that way. 

But if it's working for a group to make invis more powerful, I say go for it. There's nothing nonsensical about it.

That's the opposite of a compromise, that's a downgrade of the RAW rules. You can already attack invisible enemies, you already have disadvantage for doing so.

You just made him give up am item to follow the standard rules.

If the party is shy on spellcastwrs with the right tools...how bout a big ass bag of flour?

Another mundane way would be to use tripwires, netting, paint, or something like that.

Alternatively, you can use Darkness to put everyone back on a level playing field. I mean, if everybody including the enemy can't see, invisibility is useless. 

Web would be a good lower level way to find invisible creatures too provided you aren't in an open field.

The "cloud of flour" method wouldn't really improve the situation per RAW; unless a creature has successfully taken the Hide Action, its position is know to everyone on the field. Given the 6 second window of time combat turns happen in, you're not really going to get a better idea of a creature's position from disturbances in a cloud of flour than the "somewhere in that 5ft cube" that you already get gives you.

Really, within the scope of the initial question there's no good combat measures, because the typical issue of fighting an invisible opponent in fiction simply isn't applicable to 5e. RAW it does not take any special techniques to know their position well enough to attack, and pretty much no printed creature can effectively Hide and consistently attack in the same turn, plus Hide is not particularly reliable in itself.

AoE. You don't need to see, you just need an Invoker (or is it Evoker?). Close the doors, exclude the party from spell effects, and Fireball until nothing remains. In principle, you can also use Dissonant Whispers to herd an invisible enemy to a specific location. In the same room as above, the Bard stands in one corner, casts Dissonant Whispers at the invisible target. The target must flee to the opposite corner, which can then be blocked by spells, party members, or just blasted down with lightning bolts. Alas, Lightning Bolts no longer bounce, so that plan isn't even risky =(

Quote from PitViper0717 >>

If the party is shy on spellcastwrs with the right tools...how bout a big ass bag of flour?

Quote from The_Ace_of_Rogues >>

unless a creature has successfully taken the Hide Action, its position is know to everyone on the field.

In response to both of these posts, the context (from over five years ago before the bump) is the green hag whose invisible passage feature is more powerful than the standard invisible, condition, as described in post #14.