Voir Accel World, Season 1, Vol. 2 - Episode 7

Ruka and Mana introduce Kuroyukihime to their master, Crimson King, who turns out to be an old acquaintance of Kuroyukihime's. According to Crimson King, a very powerful Enemy is preventing them from hunting down regular Enemy units within Okinawa's field with no limits. To make matters even worse, that same Enemy is under the control of a Burst Linker calling himself Sulfur Pot, who comes to attack Kuroyukihime and the others.