I haven't found anything in game that says what these phenomena are although they clearly seem to be Falspawn related like Falmorphers and the Bayze. If I had to guess, Vizbolts seem to be the result of the F-Factor corrupting the atmosphere but what about Gehl Bulfs? Are they some out -of-phase type of Falspawn? Why do they only have a limited time of activity and why doesn't a Dark Falz like [Apprentice] try to make more frequent use of them?