D&D 5E I made a monster invisible to dark vision. Hilarity ensued.

[The Glen]

Was trying to come up with unique abilities for some custom creatures. Enter the Gaz-Thoul, a variant of the classic creature that wad more reliant on its ghoul heritage. For a kick I gave it naturally occurring invisibility to dark vision as it hides out in caves so it's hunting demi-humans normally.

Party went into the cave consisting of 1 human and 5 not humans. Two elves take watch for a rest, and the thouls attack. Everybody is scrambling to get up but nobody can see their attack despite all but one having dark vision. When the human is able to get a torch out of his pack and light it all the gaz-thouls immediately become visible. Then the party was able to defeat them with only a little difficulty, they were only CR2. Then the dwarf defiled the corpses out of spite.

Think I found a new favorite ability.

 

[Oofta]

I may have to steal this.

 

[iserith]

Cool. Don't forget that characters relying solely on darkvision have disadvantage to Wisdom (Perception) checks that rely on sight which can mean they get surprised more often. The thouls in this case might have had a good chance to attack with surprise.

 

[Zubatcarteira]

The Gloom Stalker Ranger has that feature as well.

 

[Ralif Redhammer]

I was just reading the 1979 Judges Guild adventure, Operation Ogre, and I was struck by the number of times the module included "this or that creature is invisible to infravision beyond 30 feet." From my recollection, it was not unusual to have room temperature monsters just not show up using infravision.

Now, these days, throwing a creature at the PCs that's invisible to Darkvision, that's just fiendish and I applaud you.

 

[BookTenTiger]

It would be funny if they remained invisible to characters with darkvision, even in light. The rare human or Halfling would have to constantly orient everyone else to where the monsters are.

 

[Shiroiken]

I may have to must steal this.

FTFY

 

[overgeeked]

That's great. I'm absolutely stealing this.

 

[Mind of tempest]

now this is just brutal give it some more story and we could have a classic on our hands here.

 

[Stormonu]

From my recollection, it was not unusual to have room temperature monsters just not show up using infravision.

Ah, yes. The days when undead couldn’t be seen by infravision!

I vote they should be renamed Grue-Thouls … if they continue in the dark, they may get eaten by a Grue!