R752: I came up with the following idea, but have no idea how to implement it mechanics-wise.
The Prenumbral Maw, as it is called, is one of the most rare and unusual sights upon the Material Plane, and for those who know what it is, amongst the most dreaded.
It appears as a mobile swirling black vortex. While it fluctuates in size like the shore of the ocean, on average the vortex is about the size of a humanoid, though larger one have been reported. One account even describes a cluster of them coming together and combining to form a much greater Maw, though the veracity of this report is unknown. It is believed amongst the learned arcanists that the Prenumbral Maws as mortals see them are living planar rifts, leading to an extradimensional space. Whether they are individual entities or all part of the same, much larger being is unknown; if the statement of their ability to merge is true, it would suggest the latter.
When a Maw is close to a humanoid or other suitable target, large black tentacles emerge from it, grabbing the intended victim. The similarity of these tentacles to the one produced by the eponymous spell has lead some to believe the Maw was created originally by a disasterous incident involving a botched casting of Black Tentacles. Others believe the reverse is true: the spell Black Tentacles somehow invokes and harnesses the power behind the Maws. Regardless, once the Maw has grabbed a victim, it begins to forcefully drag it into its swirling center. As a victim is pulled in, they become hysterical, dropping everything and losing their senses, only able to struggle to claw their way out. Once the Maw has swallowed a victim, it collapses in on itself and disappears. Sometimes, the victim reappears at the same spot a short time later. They describe their experience like an unremembered nightmare; horrified beyond belief but unable to recall any details. Other times, the victim is simply never seen again.
Combatting the Maw is a most difficult task. As a living planar rift, it is almost impossible to destroy, though spells such as Dimensional Anchor cause it significant distress. It is only really vulnerable when it extends its tentacles; these can be wounded like any creature. If it is sifficiently hurt, it will close up without claiming a victim.