Night of the Harbinger - S0.E2

Jack and Parker chase a fire ghost to discover a strange silvery substance, a clue that something is amiss in Newbury. Parker comes face to face with the terrifying Harbinger, who ominously tells her that 'this world is doomed', then Jack and Parker find the Harbinger as he summons the world-ending Maw, so the kids decide that it's time to end the Maw once and for all.