Samael (City of Heavenly Fire)

"However - through weakness of body... weakness of will... or weakness of trust - most of the original members we had chosen for the Dark Shadowhunters were inadequate. Thus, naturally, they never had a chance to attain their goal, yet... even this was to be expected."

- Samael was the main conspirator behind Lilith and Jonathan's Dark Shadowhunters.

Samael, also called Sammael by Lilith, is a minor antagonist but the final one in the Mortal Instruments series and a predicted minor enemy in the Dark Artifices series. Samael was said to have lived hidden as a member of the Shadowhunter Nobility all these years in Indris - the fact that he could pass through and live within the barrier is a testament to his power, he is a childhood friend of Alec Lightwood and after growing attached to this singular human he set about manipulating events to gain Alec for himself.

History
Samael was created as one of the three oldest of God's supreme creations: the Seven Archangels, Samael was given absolute dominion over the realms of Darkness, Chaos, Death and Destruction where he was given the eternal mission of destroying civilization and living beings ready for Rebirth.

When Lucifer and his band of Fallen Angels fell from Grace, God allowed Samael to do as he wished in Earth or Hell as Samael was following his pre-programmed nature: the Destruction of life and orderly civilization in preparation for a new Rebirth, when he took interest in a female Human - the first Female human - and bedded her before making her into a Demon he was forced to fight Michael and was struck down by Michael's advantage of wielding the sword "Glorious".

Clockwork Princess
While conspiring with a Demon cult that worships the Grim Reaper, Axel Mortmain sees a seemingly dead young body being sustained by powerful death magics resting in a coffin, it is confirmed that this is Samael's body. This however was revealed to be Samael's larval form, the form he takes until he regains his true power.

City of Lost Souls
Although it isn't revealed until the City of Heavenly Fire, Samael was the one who recruited Maureen Brown and gifted her with enhanced Vampiric powers comparable to the Curse of Cain, with her increased powers she easily killed Praetor Lupus member Nick and Head Vampire Lady Camille Belcourt.

Trivia

 * Samael confirms that he is a Fallen Angel, and was once an Archangel of God, third in line behind Lucifer and Michael respectively.
 * Samael reveals that he is Death, and not Death, instead he is part of the Primordial entity and yet separate from it at the same time. And, along with Michael, he is a ferrier of souls to his respective side and an enforcer of the Natural Order.