The Royal Guard (衛兵 eihei) are powerful entities within the game of Elder Tale that are charged with maintaining security and public order within cities and safe zones (戦闘行為禁止区域). They appear whenever a fight breaks out in these places, and if the players involved don't stand down, they will either be killed or taken away to a jailhouse. Guards are actually special level-100 Landers who gain their high levels through their Mobile Armor.
The Royal Guard appears as black, armored knights wearing long black capes, wielding either a battle-axe or sword. Some Guards are also seen dual-wielding their weapons. They teleport from a ball of blue light and float in mid air whenever players in cities or safe zones are about to fight. The Mobile Armor donned by the Guard are a legacy of the now-extinct Alvs.
The Royal Guard, while in their mobile suits, are capable of teleporting anywhere within Player City walls. However, these suits can only be used within those walls due to the giant magic circle underneath the ground that allow for its movement. All Royal Guards are around level 110 in order to be capable of putting down violent Adventurers.
Royal Guards are capable of monitoring the actions of Adventurers and People of the Land, but not their fellow Guards. All Guards' actions are deemed to be enforcement of security, and so any Guard that strays from their normal duty won't be detected. Because of this, Enbart Nelles' actions were not detected, and he was only discovered to be using Royal Guard armor because his set went missing.
The Royal Guards usually wield heavy melee weapons. The helmets they wear have night-vision capabilities, and the armor greatly increases physical strength as well as defense and magical resistance. However, they provide little recovery benefits since they were designed in such a way that the wearer isn't supposed to take damage to begin with. It also seems that one does not need to wear the full suit of armor in order to gain its effects; Nelles only wears part of the helmet, the bracers, and the shin guards and seems to have all its abilities.
Purpose and Flaws
The Royal Guard act as peacekeepers within the within cities and safe zones that are in charge with where, they will, almost without fail, punish anyone who attempts to start a fight, steal, or keep someone prisoner within a noncombat zone. If an offending player was detected by the Royal Guard, they are either placed into a prison at an unknown location for a period of time before being released, or killed by the Guard and will revive in the Cathedral.
However, it is designed to work only on actual combat actions, such as using a weapon to attack another Adventurer or Lander, casting a combat spell against another player, or intentionally inflicting HP damage. As a result, certain actions that only became (reasonably) possible after the Catastrophe, like harassment, enslavement, and other non-combat moves are not in the Guards' detection capabilities. Furthermore, in cases like poisoning or dropping something from a long distance, it may not register as an attack depending on the circumstances and intent.
In episode 2, The Royal Guard was mentioned when Shiroe and Maryelle discuss the rampant Player Killing, with a visual of three of Royal Guards appearing out of nowhere to quell the fight between Adventurers.
Fallen Guardian arc
Enbart Nelles becomes possessed by the sword that contained the soul of an Ancient Raid Boss, Lugrius. He kills West Wind Brigade member Kyouko, inciting the rage of Soujiro. For the next week, this "Murderer" kills several Adventurers, including a patrol team from Honesty. In an attempt to defeat it, both Soujiro and later Akatsuki die attacking the Raid Boss-Guard.
Later, a member of the Kunie clan, Kinjo, approaches Rayneshia with the news that a Guard had stolen one of the mobile suits. Because of the possessed Guard's ability to use the teleportation system to escape pursuers and find new targets, the Round Table members present are forced to shut off Akiba's Royal Guard System. Not only did this stop the Royal Guards from appearing in case people fought within city walls, it also meant that monsters, which were usually kept from entering the city by the Guards, were now free to enter the city unless stopped by Adventurers themselves.
In the Log Horizon: West Wind Brigade spin-off manga, Magus, Passhita, and Coza take advantage of this knowledge to sexually assault Isami and Sara. Because they weren't doing anything that dealt damage, the Royal Guard didn't respond to their actions, and only showed up after Isami attacked Passhita with her sword and drew blood. Soujiro, who appears right as the Guard does, decides to draw the Guard's attention to himself to buy Isami time to escape. Although he sacrifices his own life in exchange, he revives in the Cathedral and the Guards seem to drop her case some time after Souji's death.