Can a monster with tremorsense that is blinded make attacks of opportunity when an enemy leaves its reach?
Tremorsense is described as:
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 sense.
There is a Sage Advice ruling on blind opportunity attacks that specifically mentions blindsight but does not mention tremorsense
An opportunity attack is triggered by “a hostile creature you can see”. If you can’t see an enemy, you can’t make an opportunity attack against it.
Creatures with blindsight are an exception to this rule, because that ability lets those creatures “see” within a certain radius.
As long as the target is touching the ground does tremorsense essentially work the same as blindsight with regards to opportunity attacks?
Best Answer
The blinded condition does not impede tremorsense
Nothing about tremorsense involves vision at all. Tremorsense specifically functions by sensing vibrations transmitted through the ground. Hence, the blinded condition would not have any effect on the ability of a creature to use or benefit from its tremorsense.
Ok, but what about attacks of opportunity?
The above answer is simple and straightforward, but unfortunately it doesn't fully answer the question. The problem here is that the description of tremorsense doesn't describe the degree to which the sense can substitute for vision in the first place. The only specific mechanical effect described is being able to pinpoint the location of any creature within range. Complicating matters is the fact that, as far as I can tell, all monsters with tremorsense also have darkvision with at least the same range, so cases where a monster can sense a creature with tremorsense and attack that creature without also being able to see it are vanishingly rare, at least until the PCs start pulling out blinding effects and force their DM to make a ruling.
So my recommendation for a DM is this: make a ruling on how tremorsense works in your game. There are 2 logical rulings that I can see based on the description:
If you decide on the first ruling, then blindness, magical darkness, and any other effect that impedes vision have no detrimental effects on a creature with tremorsense, as long as the creature's opponent is detectable via tremorsense (i.e. not ethereal, flying, etc.). With this ruling, a creature with tremorsense can make attacks of opportunity as normal even while blinded against targets it can detect with tremorsense. Or to put it another way, sensing a creature via tremorsense is mechanically equivalent to seeing it with normal vision.
If you decide on ruling 2, then a creature with tremorsense in blinding conditions suffers most of the normal detriments of blindness, including disadvantage on attacks, advantage on attacks against it, and being unable to make attacks of opportunity. The only exception is that blindness does not prevent a creature with tremorsense from knowing the location of detectable creatures. Hence, it would not be possible to hide from it (using the hide action), since it would still know your location (just like it would not be possible to hide from it by getting behind total cover).
Ultimately, I can think of reasonable arguments for both rulings, so I'm not going to recommend one over the other. However, I believe that ruling 1 is much more common in actual play, not least because it makes monsters with tremorsense more of a threat and makes them easier for the DM to manage in battle (one less source of advantage/disadvantage to keep track of).