The undead are a frustrated bunch. They want to let their weary souls rest, and yet the current administration of King’s Landing is bent on preventing them from reaching the magical pool in the depths of the Red Keep where they can drown their misery permanently.
Lucky for these tortured, half rotten lot, the new king Tommen has resolved that he will read an incantation at waters edge which will allow the zombies to dissolve themselves in the dark water. Cersei takes issue with this, and sliding down the catacomb stairs at an alarming rate, she tries to knock the book from Tommen's hand. Instead she ends up falling into the water where there razor lined tentacles of strange squids whip about hungrily.
As legions of zombies begin willfully plunging themselves into the waters to be nullified, Cersei looks on for a horrified, reluctant moment before being consumed by the nearest hell-squid.