Nimroth is the father of Nimiane, the main antagonist of the 100 Cupboards trilogy. He is also the former emperor of Endor, the founder of the undying race, and the first known person to exploit the power of the Blackstar. He was driven to insanity after the faeren stole the Blackstar from him and was eventually sealed away inside a crypt by his sons. Prior to his downfall, he was an extremely powerful man and greatly feared.
Nimroth is alternatively known as "Nimroth the Devouring" and the "Blackstar." 
Nimroth was a seventh son and a green man, connected to poison vines. He was a learned man with a large library of his own. He lived in his father's house before Endor fell into evil and developed a taste for wizardry while he was still young.
Nimroth was not content with his mind or body. He searched the world for a prison of incubi and eventually came upon the Blackstar. Using the Blackstar, he imprisoned incubi in his blood making himself immortal, half-human, and very powerful. From then on, his life depended on the life of others, which he took by "drinking" them.
He gave incubi blood to his "loved ones" and thus started a new race of undying Endorians. Princes and queens came to Endor in order to buy incubi blood from Nimroth, willing to pay anything to get it. Nimroth made himself emperor of Endor and expanded his kingdom through the power of the Blackstar.
Recognizing Endor's threat, Nimroth's evil, and the centrality of the Blackstar to his power, four faeren infiltrated Endor in an effort to steal the Blackstar. Their efforts were successful but only one of them, Nudd the Chestnut King, survived.
Nimroth denied to his court that the Blackstar had been stolen and even disguised an ordinary stone as the Blackstar in order to convince them that this was true. Nevertheless, he searched relentlessly for the thief, losing his sanity in the process. In his madness, he became a ruthless and abusive ruler and convinced himself that the fake Blackstar was actually real. He tortured and imprisoned his own family members and executed all of the mortal servants in his palace.
Endor fell into madness after this and many lost their minds. "Tomb cities" were built underground for those who had become insane. In them, the mad Endorians were sealed away in crypts and coffins, similar to how the incubi were once sealed. Nimroth himself was eventually imprisoned by his sons in one of the deepest levels of these crypts, and his fake Blackstar was sealed away with him.
In The Chestnut King, Nia transforms her physical appearance into that of Nimroth during his reign. Through Nia, Nimroth is depicted in this scene as a tall, broad-shouldered man with black hair and sharp eyes. He is seen (through Nia) wearing on his head a silver band studded with swirling stones. At one point, he is holding a gold mace. At another point he is holding three serpents in one hand and the Blackstar in the other hand.
- "Nimroth the Devouring, Blackstar…"—Nimroth, p. 69 The Chestnut King hardcover edition.