How to make an ability to sense vibrations in the ground? [closed]

Question

Am pretty far along on baby's first homebrew race, but I've run into a hiccup: These are grub-people with an uncanny knack for feeling vibrations of the earth, even if they are above ground. (a la blindsense) If I wanted to make this a viable racial trait/ability, how would I make it functional? If you need any additional info, please ask.

NOTE: Tremorsense and Blindsense are both pretty good, but I found a good way to do it (unfortunately can't think of a good name, I thought tremor sense was a good name until I learned it was already a thing). Thanks to everyone who answered! ^^

When an unseen object or creature within 30 feet above ground or 20 feet underground is approaching on foot or via burrowing underground, the player should be asked to roll a perception check. If this check succeeds, the PC now knows that something is approaching, but has no additional information such as size or number of approaching objects or creatures. Then the player may roll an additional perception check for honing their to determine either a rough estimate of the approaching force’s size or numbers.

Answer 1

You want Tremorsense

The ability you're looking for already exists; it's called Tremorsense:

A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.

Tremorsense can't be used to detect flying or incorporeal creatures. Many burrowing creatures, such as ankhegs, have this special ability.

You don't want Tremorsense

As a monster ability, it's pretty scary because it doesn't require vision and hiding doesn't help. For monsters that will appear in an encounter or two, it's not that big a deal because it is temporary. The monsters will die and the game will move on.

As a player character ability, I'd consider it almost broken-powerful. It will have an impact on almost every single encounter, rendering many forms of sneaking, stealth, and surprise utterly ineffective against the party. Granting this ability as a racial ability will make it available at level 1.

It also steps all over a Rogue class ability Blindsense (different from Blindsight), which makes the Rogue automatically aware of hidden and invisible creatures, but only out to a range of 10'. A Rogue does not get this ability until Level 14.

If you absolutely insist on giving it to a race, keep the range very short, certainly no further than the Rogue's Blindsense.

A Possible Alternative

Perhaps instead of full-up Tremorsense, you could grant this race a lesser form that gives advantage on Wisdom (Perception) tests against the same sorts of creatures. It would allow the ability to get better over time (as the character's proficiency bonus and Wisdom potential improve), rather than being automatic and perfect from the get go.

Answer 2

On page 9 of the MM it talks about Tremorsense

A monster with tremorsense can detect and pinpoint the origin of vibrations within a specific radius, provided that the monster and the source of the vibrations are in contact with the same ground or substance.

Starting on page 285 in the DMG is a section detailing how to create a new race. There are different sites that help guide the race creation process as well.

Answer 3

What you're looking for is something along the lines of tremorsense:

Tremorsense was a type of sense that a few kinds of creatures had. Creatures with such a sense could detect and precisely locate vibrations, including those of other creatures. Creatures with tremorsense had to be in contact with the source of the vibrations, that is, via the ground, thus it couldn't be used to detect flying or incorporeal creatures.

The general way this works is that you give a creature a tremorsense radius as a type of sight, in the same way you might give a creature dark vision, and that creature can sense the location of any other creature within that radius when it is in contact with the floor. Flying or hovering creatures are usually immune to being detected in that way.

A more general sense is blindsight, which allows a creature to use a non-sight sense in order to detect and/or locate other creatures. Tremorsense is arguably a type of blindsight, but other examples include echolocation and improved hearing.

For your specific case, you might consider basing your grub-people off the Black Pudding monster, although you'll need to adjust the abilities and damages to fit the kind of CR you're looking for.