Fun fact: It is unknown how the Overlord converted itself into a virus, but considering it’s role as a source of all darkness in Ninjago, it may have simply recovered after it’s defeat, taking his digital form to adapt to the increased technological progress of New Ninjago City. It is also implied that Cyrus's construction of Borg Tower over the remains of the Garmatron may have reawakened the remnants of the Overlord's being, which merged with Borg's technology to regain his strength.