I'm running a campaign that the players are about to encounter several burrowing creatures. The creatures in question do have Tremorsense so they'll be able to tell where the players are and with good burrowing speed should be capable of ambushing them. I'm just curious about two things:

  1. Can I roll Stealth (or do I even need to) to have them hidden underground before jumping out and attacking. Hopefully causing players to be surprised.
  2. Since they are entirely hidden beneath the ground, does that mean their first strike burrowing out of the ground counts as hidden and gives them advantage?
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The attacker should roll a Stealth check with advantage.

The burrowing monster is hiding (under the ground) and heavily obscured (because the ground is in the way). Perception checks to detect a heavily obscured creature have disadvantage.. You can either let the burrower roll Stealth with advantage or give the victim -5 on their passive Perception to detect the ambush. If the burrower wins, they'll have surprise during their attack.

Note that, surprise or not, the burrower and the guy on the surface still can't see each other until the burrower comes out of the ground.

The attack will have neither advantage nor disadvantage.

Depending on the monster's anatomy, either it can attack before digging through the surface (say by stabbing a big stinger or horn up out of the ground) or it can't. If it can't, then it has to come up first, and then the attack is resolved normally.

If it can attack while burrowed, then it's still locating its victim by tremorsense alone, which is not enough to aim accurately*, but the victim also can't see the attack coming. As usual when the attacker and defender can't see each other, advantage and disadvantage cancel out. This is technically different from the above case in that it also negates other sources of advantage on either side.

About tremorsense

The question has been raised: is tremorsense accurate enough to aim an attack? I think it's not. The rule here is that "When you attack a target that you can't see, you have disadvantage on the attack roll."

Tremorsense doesn't say that it lets the creature see (like Darkvision and Truesight), nor that the creature doesn't need to see (like Blindsight). It's also not mentioned in the Vision and Light rules, a strong clue that it's not a kind of sight.

  • \$\begingroup\$ But there are monsters with only tremorsense that don't say they attack at disadvantage :) \$\endgroup\$ – NautArch Jul 12 '19 at 1:18
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    \$\begingroup\$ @NautArch Why would the rules need to explicitly say they attack at disadvantage? The section on Unseen Attackers And Targets already says it. \$\endgroup\$ – Willem Renzema Jul 12 '19 at 1:28
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    \$\begingroup\$ Unless the monster's description says that they're blind or eyeless or something, I'm going to assume they can also see light. It's similar to how there's a "Blinded" condition that means you can't see, but no condition for when you can see. \$\endgroup\$ – Mark Wells Jul 12 '19 at 2:59
  • \$\begingroup\$ Not being listed under the Vision section isn't just a "clue", it's explicit that tremorsense isn't sight. This is one of the big differences between tremorsense and blindsight. \$\endgroup\$ – user-024673 Nov 15 '19 at 9:00
  • \$\begingroup\$ @jgn Not being listed somewhere can't be "explicit" anything. "Explicit" would be if Tremorsense said "This isn't a form of sight and doesn't substitute for being able to see." \$\endgroup\$ – Mark Wells Nov 15 '19 at 14:43
  1. You can roll a stealth check against their passive Perception -- but don't have to. You, the DM, decide when a character is surprised:

    The DM determines who might be surprised.

  2. Yes, your monsters would qualify as unseen attackers if they are hidden (should require a Stealth check), and gain advantage on the first strike.

    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.

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    \$\begingroup\$ Unseen is not synonymous with hidden. Just being unseen (but not hidden) is enough to gain advantage on an attack. However, I'm not sure a burrowing creature can attack from underground. If it must surface before attacking, it wouldn't be unseen when it makes the attack, right? \$\endgroup\$ – Ryan C. Thompson Jul 11 '19 at 20:30
  • \$\begingroup\$ @RyanThompson Usually, the unseen bit happens after the first attack, no? A creature with invisibility still gets their first attack at advantage before the spell turns off. \$\endgroup\$ – NautArch Jul 11 '19 at 20:34
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    \$\begingroup\$ @NautArch That's true for invisibility, because the attacker is still invisible when they make the attack. I'm saying that I don't know how that works for burrowing. Maybe that's the basis for a separate question entirely - does a burrowing creature need to surface before attacking? \$\endgroup\$ – Ryan C. Thompson Jul 11 '19 at 20:52
  • \$\begingroup\$ @RyanThompson I'd argue that until they surface, any possible target has total cover, but that might be worth taking to a separate question. \$\endgroup\$ – Mark Wells Jul 11 '19 at 21:09
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    \$\begingroup\$ An underground creature with Tremorsense should know where its targets are, and should be able to attack them from underground, depending on what attack options are available; claw attacks should work, bites not so much, etc. \$\endgroup\$ – Journer Jul 11 '19 at 22:02

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