The Bannus was inactive for many years and sealed with the mark of the Reigners so that no-one dared turn it on. During the events of Hexwood, Hexwood Farm employed a clerk from Earth who didn't realise the danger and turned on the machine to play simulation football on. The machine quickly went out of control.
The Bannus seems to be semi-intelligent, and is running its program for some purpose, but it is not clear what.
There are spoilers connected with the nature of the Bannus (see [Hexwood Spoilers]?)