The Divine Focus was a weapon found atop Armadyl's Tower near the Clan Camp. The god of justice Armadyl used this weapon to slay the god of war, Bandos. Armadyl's head diviner, Kamiee, describes the weapon as a "building-sized ritual to stop Bandos. A war-ender."
A divine transmitter at the bottom of the tower transmitted a beam of energy up towards the focus itself, which was then presumably stored inside it. It was powered by divine energy, which Armadyl and Kamiee collected by sending out caravans to sites around Asgarnia and Misthalin. At the sites, the caravans used Divination to siphon energy wisps, creatures created from the Anima Mundi that were created as a result of Guthix's death. When enough divine energy was collected, the Divine Focus redirected the beam of energy shot from the transmitter to Bandos's Tower.
Kamiee fires the Divine Focus.
Bandos adopted similar tactics, constructing a divine energy-powered weapon called "the Scarecrow". However, the Bandosian followers gathered less divine energy than the Armadylean followers, enabling Armadyl to fire his weapon first. This decapitated Bandos, destroying his tower and the Scarecrow.
The Divine transmitter beams energy to the Focus.