Random dashes through to helter-skelter running through house in pursuit of unseen prey. Cats suffering from greeblingz typically have wild-eyed expressions. A very few cats are naturally immune. Anti-epileptics are ineffective as the condition appears unrelated to other forms of seizure. Avoid getting in the way of a cat engaged in greebling hunting.
Attacks usually subside spontaneously, perhaps as greeblingz return to their own dimension.
These irritating creatures are not visible to human eyes, but no doubt the superior sight and hearing of cats enables them to see them.