Essyllt Iorwerth, also known as the Head Mourner, was the commander of the Prifddinas Death Guard and a member of the Iorwerth Clan. He was one of Lord Iorwerth's highest officers who sought to find an entrance to the Temple of Light in order to harvest its power to bring back the Dark Lord. To this end, Iorwerth convinced King Lathas to fake a plague in West Ardougne so that the mourners could quarantine the area and thereby have free access to the tunnels which led to the temple. The head mourner was handed the duty of capturing people who fell ill to the "plague" and then using them as slaves to dig through the tunnels to find the temple. Essyllt is also part of an elven warband, and makes a brief appearance alongside Morvran during Regicide to kill Idris, one of the elven rebels.
Essyllt ordered Elena's imprisonment after learning that she was investigating West Ardougne for details of the plague supposedly brought on by the return of King Tyras. The adventurer rescues Elena and, together with her mentor Guidor in Varrock, they discover that the plague was a lie. Eventually, the player infiltrates the mourners as a new recruit in Mourning's Ends Part I. The head mourner tasks the player with interrogating a gnome the mourners captured to fix a device (normally used to make sheep look plagued). After fixing the device, the player spreads the "plague" across West Ardougne and completes the rest of the tasks Essyllt assigns them.
Pleased with this progress, the head mourner tasks the player with acquiring the power at the Temple of Light, in order to speed up the Dark Lord's return. Instead, unbeknownst to Essyllt, the player re-activates the temple, shutting off all access to the temple for the next thousand years.
After the Temple of Light was sealed off, the head mourner's mission became the extermination of all the citizens of West Ardougne, with Lord Iorwerth telling him to kill the civilians however he wished. During Within the Light, the player has the option to talk to him regarding the gnome inventor and to persuade him to consider releasing the captive. The gnome's fate is not revealed until Plague's End, where Essyllt informs the player that - after much consideration - he set the gnome free by having him blindfolded and led out of mourner headquarters. If the player did not speak to Essyllt on the gnome's behalf, he instead reveals that he considered the gnome to be too much of a risk and killed him via poisoned toad crunchies.
During the Ardougne Revolution, the revolutionaries storm the headquarters and kill the mourners inside. Essyllt fights the player personally but is eventually defeated and begs to be spared. He claims that he has a wife and child, and that he was just following orders. The player can then interrogate him regarding how he wanted to kill the population of West Ardougne and the gnome. Essyllt claims that he would give the citizens a "quick and merciful" death by herding them into a cavern and collapsing the roof on top of them. He can be either killed or spared. If spared, he leaves and returns to Prifddinas.
It appears that Essyllt lies often: prior to joining the Death Guard, he told his wife, Mwynen, that he was an emissary of peace for the former Lord Iorwerth. If Essyllt is spared, he continues to uphold this lie in Prifddinas and the adventurer has the choice to either play along, quietly receiving Essyllt's gratitude, or to confront him about it, at which point his wife, shocked and upset, will request to speak with him in private. During Plague's End, he says he has both a wife and children but, if he is spared, only his wife is with him in Prifddinas. When the player asks him about a child, he attempts to lie again by saying that one of them went into the underground pass, another is cleaning up the undercity, and the last one is wandering around. The adventurer has the option to ask him about the last child, whose name he will repeat incorrectly. If the adventurer does ask about the child, Essyllt will admit that he doesn't actually have any children; if the adventurer does not do so, no more will be said but his wife will glare at him menacingly, clearly indicating that something is amiss in his description of their alleged children.
Prior to Plague's End, the head mourner can be found in his office in the basement of the mourner HQ in north-eastern West Ardougne. Mourner gear must be worn in order to enter the building. If lost, two keys can be found on his desk: a tarnished key to the jail room next door and a new key to open the western basement door to the tunnel leading to the Temple of Light.
After Plague's End, if he was spared during the quest, he may be found alongside his wife, Mwynen, within the Iorwerth manor in Prifddinas.