D&D 5E Tremorsense

[Saeviomagy]

Personally I really like the idea of the aliens motion detector. It seems like that adds something interesting to the game.

I would probably say that motionless characters are not detected, because that fits with numerous sources of fiction for similar abilities (dune worms, worms from tremors, alien motion detector) and makes tremorsense an interesting addition to a game rather than just being 'like seeing, but ignores invisibility and stealth'.

 

[Rya.Reisender]

If the target is successfully hiding, I'd say it manages to not be detected. I'd give advantage on perception checks when the hidden target is moving. Not hidden... well it will almost always be detected, because you will always move if you don't pay attention on not to do it. Even if you sit at a table with some allies, you'll move a lot.

Edit: To clarify, if someone said "I stay still to not be detected by tremorsense", that would be a stealth check for me.

 

[Dale Hartley]

We play it that a land bound corporeal creature in range is always detected by a creature with tremorsense regardless of surface just like a PC can always detect a visible creature in a lit room. It keeps the game simple. Remember that just because a creature can see you doesn’t mean it can attack you. Being halfway up a cliff or wall can still help if the creature can’t climb.

We have had PCs try to use stealth to get around it but the ruling has been to do this they have to declare they are using it specifically against tremorsense (so it doesn’t apply against other senses because you are focusing on not making vibrations instead of being seen or heard) and have the ability to hide in plain sight to move and not do the check at disadvantage. We make the same requirement for a PC to be still enough to not be “seen” while not moving or taking actions.

We have also tried using magical items to disrupt tremorsense by generating powerful vibrations that act like bright lights or loud noise to provide cover for the much quieter “noise” of PCs running around. This usually results in the creature attacking the magic item and swallowing the vibration source. How this affects the creature depends on the DM but we have had it nuturalise or amplify the effect for the creature based on whether it got destroyed by the thing chewing it. This is definitely not a standard item.

There is an argument that a creature with a finely tuned enough tremorsense could perceive the very vibrations of the universe (think atoms sub atomic particles etc) but this isn’t necessary for the game to work and the fact that it cannot detect flying or incorporeal creatures says that this normally is not what is happening. If a creature did have this it would be the equivalent of granting it x-ray truesight. So I’m thinking primordial or god.

So pretty much these days we just play it simple as per the first paragraph. After all this is a game and the point is to entertain, not debate fine points of quantum mechanics.