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. 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 plans of the Bannus (see Hexwood (spoilers))