Return of the Goblin King is the sixteenth episode of the Log Horizon anime. It covers Game's End, Part 2 chapter 1, and a beginning portion of chapter 2.
Plot
Cold Open
At the beach, the battle against the Sahuagin continues. The trainers form a pick-up group, trying to cooperate on the defense. Kawara uses「Tiger Echo Fist」on several Sahuagin and Shigeru asks her how long it would take before she could cast it again, bemoaning that he couldn't aggro all the Sahuagin in the area. Shouryuu and the other trainers stand back to back as they become surrounded by Sahuagin, hoping to delay the monsters long enough for Marielle and the low-level players to escape to the school.
Goblin Problem
At Ragranda Forest, the camp begins encountering an unusual amount of goblins inland, and Nyanta oversees a massive goblin army approaching from the north. Word quickly reaches the palace, while Nyanta reports back with to camp with an assessment of the army. The trainers don't have telepathic links with the Crescent Moon to check on the other training group, but Touya's party does, as they return from fighting goblin scout patrols. Naotsugu is surprised to learn about the Sahuagin attack, whom usually reside further south. The trainers decide to regroup with Marielle and hold the Call to Home spell as a last resort.
The Goblin King is crowned.
After pooling their knowledge, Naotsugu informs Shiroe who organizes an emergency relay conference with the Round Table Alliance. Shiroe reports that an estimate 10000 goblins and several hundred Sahuagin have appeared, and postulates that the recent attacks are part of the quest Return of the Goblin King. The Adventurers then realize that they neglected quests from the People of the Land to defeat the goblins during their development of Akihabara, which in turn allowed the Goblin King to unite a great army. This is no match for the adventurers, but Eastal will suffer greatly from conflict with this enemy.
Meanwhile, Rayneshia's father announces to her that he must leave to defend the town of Maihama from the goblin invasion, ordering her to stay in the palace to help Sejiad determine a course of action.
Choushi's Jeopardy
The forest trainers have reunited with Marielle, but soon notice a goblin raiding force marching towards the nearby town of Choushi. With barely any defenses, Touya and Rudy request that their party defend the town, but everyone else is hesitant for the adventurers to intervene.
Shiroe summons Soujirou and the West Wind Brigade to the Ancient Palace of Eternal Ice to aid in its defense should it come under attack. He then informs the Round Table Representatives of his hypothesis that a death induces memory loss, which Krusty verifies and shockingly reveals that he had already lost some memories of the real world.
Elsewhere, Minori persuades the trainers to let her party deploy as scouts, only to take off to defend the town. They come across a Lander house that is being raided by goblins, with Touya attacking the goblins after realizing they knew the residents. Touya and Minori reckon that they shouldn't wait for Landers to ask them for help, and that the will to help is good enough reason to save the town. Nyanta and Naotsugu agree to this as they appear with Shouryuu, concurring that adventurers are free to do what they think is best. Minori decides that since they are not strong enough to defend the town, they will attack the goblins directly instead.