This is a place where the numerous aliens, demons, monsters and other unsettling or unearthly oddities I use in my work will be documented in a hopefully engaging fashion, so that if you see one, you will know that the universe has finally become a madhouse. Oh wait...
The Timeworms know because they find the doors. They swim and squirm and float like cataracts through the many levels of spacetime, consumed by destructive secrets they are not allowed to tell without inflicting madness in the process.
Described as clusters of thin faintly luminous white worms that drift together through the air, giggling and shrieking like giddy children, the Timeworms are thought to possess knowledge of the past as well as the future and perhaps timelines from other universes entirely, which they seem obsessed with telling to those living on the physical plane, especially humans, curious by nature.
Encounters with Timeworms are hard to distinguish from ordinary cases of mental illness, but the afflicted will occasionally be coherent enough to share bits and pieces of what they heard from the tormenting travelers, not enough to enlighten; just enough to unnerve and confuse the recipients of this overloading alien intellect.
They are the only known beings that can accomplish time travel through a material world without being stopped in their tracks by the laws of physics, and only because they can have no effect on the realities they end up in, aside from than the secrets they share. Secrets that almost always cause the listener to lapse into insanity or wordless catatonia, perhaps to preserve some universal law that forbids any mortal being from affecting reality in ways other than intended.
Timeworms are also best described as mischievous, almost playful in their insistence. No decipherable words have been heard spoken by them, but they seem to politely demand that the listener pay attention.
When one reaches out to touch these creatures, they seem only half solid. One can feel a quick slithering coolness where they swim through the air but cannot grab hold of their thin bodies or harm them in any way. And vice versa.
The few that encounter Timeworms have their awareness expanded by universal understanding beyond the scale of human thought process, awareness that swiftly becomes to much for them. It could even be possible that the truths we could gain from them are not truths at all, at least in our world.
And after the mad potential prophets are carted away to their local mental institute, the gossipy abominations depart thereafter, as though a mission has been fulfilled.
If by any chance someone does not go mad from knowing what is not meant to be known, they have a tendency to disappear without a trace. Taken? Dead? Perhaps that too is a secret not worth telling.