Tremor Sense

A creature with Tremor Sense is sensitive to vibrations in the ground and can automatically pinpoint the location of anything nearby. This ability functions at a shorter distance than vision range but it is still impossible to sneak by such a creature guarding a hallway. This makes using stealth to complete quests with monsters that have Tremor Sense problematic.

Certain rare effects that cause a character to float (such as the Pale Master's Shroud of the Wraith) also make them invisible to Tremor Sense, allowing Stealth even in these situations. This also applies to summoned creatures with Airborne, although they will sadly squander their advantage by recklessly attacking.

Monsters
At this time, it seems that only spiders (both the Vermin and Magical Beast variety) and Oozes have this property in DDO. Scorpions (both the Vermin and the Magical Beast variety) and umber hulks do not act like they have Tremor Sense. even though they do so in PnP.

Mechanics
In DDO, this trait provides the following benefits:
 * The creature automatically passes all Listen checks. If they don't have line of sight or are outside of Tremor Sense range, they begin investigating the area. This is represented by a large red question mark above their head.
 * The creature automatically passes all Spot checks the moment they are close enough and line of sight has been achieved. This is represented by a large red eye above their head.
 * The creature bypasses all miss chances due to concealment from things like Displacement but not from those caused by the cloud types spells (such as Stinking Cloud).
 * The creature can detect and locate invisible creatures at no penalty. Invisibility provides no benefit against them.
 * The creature does not suffer the 50% miss chance while blinded. However, blinding them will still cause them to wildly attack around, as if they can't locate their target. They will also be susceptible to sneak attacks.
 * Against floating enemies, a creature with Tremor Sense does not auto-pass Listen and Spot checks and cannot bypass Invisibility. The other benefits still apply.