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The Invisibility condition states:

For the purpose of Hiding, the creature is heavily obscured.

Rules for The Environment state:

A heavily obscured area—such as Darkness, opaque fog, or dense foliage—blocks vision entirely. A creature effectively suffers from the Blinded condition (see Conditions ) when trying to see something in that area.

The Blinded condition states:

A blinded creature can’t see and automatically fails any ability check that requires sight.

A group of 3 characters is under the effects of Great Invisibility and takes the Hide action. Within a minute later, a group of hostile npcs (who do not have any abilities above darkvision and hearing) enters the area. Do they automatically fail all perception checks (thereby granting the invisible party a surprise round)?

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marked as duplicate by Wyrmwood, KorvinStarmast dnd-5e Mar 22 at 20:57

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The hostile NPCs would not "auto-fail" their Perception Checks

Perception to locate a hidden creature is not solely about visually spotting them. It's also about hearing them (they still have to breathe, they might flinch or bump into something), or seeing evidence of their presence (maybe a bootprint on the ground, or a spot where the dust has been disturbed?). Bear in mind that ability checks are intended to be abstractions of a character trying to do something. You can tell your DM "I don't make any noise", but if your character breathes too loudly, or if the acoustics of the room are just right (Diegetically: you rolled poorly on your Stealth Check) then you might still get detected.

So if the NPCs walk into this room and aren't already aware of the PCs' presence, then their Passive Perception would be used to attempt to detect the party. If they are already aware of the party (like if they saw them run into the room before entering, or otherwise suspect someone is in the room) then they'd make active perception checks.

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  • \$\begingroup\$ So the party would need to also have Silence and/or Pass without Trace active to prevent sound and make the checks auto fail? \$\endgroup\$ – Rykara Mar 22 at 20:33
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    \$\begingroup\$ @Rykara Yes; although in practice, the +10 bonus from Pass without Trace would probably be good enough as-is, and the Silence effect would probably be redundant. \$\endgroup\$ – Xirema Mar 22 at 20:34
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    \$\begingroup\$ @Rykara That would help, but even then you can still be found by things relying on other senses, like a dog with a strong sense of smell, or anything with tremorsense. You'll still be incredibly hard to find though! \$\endgroup\$ – MrSpudtastic Mar 22 at 20:36
  • \$\begingroup\$ @MrSpudtastic Also a good point! \$\endgroup\$ – Xirema Mar 22 at 20:37
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    \$\begingroup\$ This answer would be greatly improved if it detailed the difference between hidden and invisible. I can be hidden and therefore, effectively invisible, but I can also be invisible and not hidden. \$\endgroup\$ – Wyrmwood Mar 22 at 20:41

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