No Overlord has yet realized that there is another library. When or if one of them does find the other library, the results aren't likely to be happy, at least not for Gnarl.
The other library has always been in the Netherworld. It contains Gnarl's collection of research on minion mythology, his notes on Tower management, and his chronicles of all the Overlords, complete with his honest assessment of said Overlords' abilities.
Since no Overlord has ever been particularly mild-tempered, the result of any of them ever seeing Gnarl's notes is likely to be... fiery.
Fortunately for the old Minion Master, there is only one Overlord who is likely to ask, and he is currently trapped in the Infernal Abyss.
Gnarl is not the first minion ever spawned, but he is one of the first generation, and the oldest minion alive. Mortis is from his generation, although spawned some years later. Giblet was once trained to follow Gnarl as a Minion Chieftain, a position that was later abandoned when Gnarl proved capable of managing all the minion tribes.
Despite his extreme age and apparent frailty, no minion will fight Gnarl if he can avoid it. While the Minion Master isn't as strong or as fast as he was, he's skilled enough that when anything tries to attack him, he's somewhere else by the time the attack hits. He can read intention from body language so well the other minions think he reads minds. He might: Gnarl is very cagy about exactly what he can do.
Gnarl has forgotten much of what is preserved in the Netherworld library. Partly this is because of the many thousands of years that have passed since he wrote the first chronicles, and partly it's related to the curse on the Forgotten God.
The first of the Overlords, hundreds of years before the Black Baron and Lord Gromgard, was the son of the Forgotten God and the elf woman the God dallied with, a priestess of the God in His aspect as the Father of All. Back then, almost as many elves worshiped Him as the Father as worshiped the Mother Goddess. The God's elves paid a terrible price: every male elf was transformed into the demonic creatures we know as minions and banished to the Abysses. The transformation left them with no memory of what they had once been and a remnant hatred of elves.
Every true Overlord since the first carries the blood of the Forgotten God. It breeds true, although sometimes the God's nature can be very deeply buried. Arguably the biggest mistake made by Overlord Mardis (popularly known due to a lack of historical knowledge as the Second Overlord) was to choose a man with Overlord ancestry as a temporary substitute. It should surprise no-one that his substitute, Overlord Doniel, ultimately defeated him before being lost to the Infernal Abyss.
No Overlord has ever died a natural death, although it's possible that due to the nature of the Overlords they consider death by crushing, dismemberment, immolation, and in extreme cases giant weasel to be 'natural'. All of them until Overlord Doniel did actually die. This creates a problem Gnarl has yet to mention to Doniel's son Overlord Thorn not that Gnarl is actually lying, exactly. He simply has a policy that it's better not to mention things that might never happen, since it's usually easier to explain how something wasn't his fault after the fact than it is to explain things beforehand. It saves awkward questions.
Unlike the other minions, Gnarl's loyalty is to Evil. Originally he was loyal to the Forgotten God, but since he too has succumbed to the Mother Goddess's curse, he's loyal to the closest possible alternative. He even says this to every Overlord. Most of them assume that means he's loyal to them as well, not realizing that Gnarl is loyal only as far as it serves Evil. Should he decide that an Overlord is doomed, he will quietly arrange for the preservation of the most important assets of the Tower (usually not the ones that get looted on a regular basis: the important assets are things the Overlord rarely knows about since in Gnarl's opinion there's no need for an Overlord to dirty his gauntlets with the minutiae of running an Evil Domain).