This node refers to the fiction associated with the game Warhammer 40K.
Mephiston is the Chief Librarian of the Blood Angels Space Marines Chapter. He is also known as the Lord of Death. He is an awesome figure of power, and a presence on the battlefield to be reckoned with.
Mephiston was originally named Calistarius, and joined the ranks of the Blood Angels Librarius. It's worth noting here that being a Space Marine Librarian isn't about looking after books. The Librarius is the psychic contigent of a Chapter, and are called upon to protect the other Space Marines from attempted incursions by Chaos powers, and sometimes to go into battle, using their unique powers to aid the cause.
Brother Calistarius was a fairly good Librarian, but what he was really known for was a strength of character. During a prolonged battle, he fell victim to the Red Thirst (see Blood Angels for more information). Calistarius was immediately made a member of the Death Company, so that he might end his life by dying honourably in battle, rather than descending into unholy madness.
He joined a Death Company assault, and, along with many others, was trapped under a collapsing building. By rights he should have died there, before or after succumbing completely to the Red Thirst. But somehow, he was able to defeat the Thirst. No other Blood Angel has ever achieved this incredible feat. In overcoming the Red Thirst, Calistarius became far more powerful, having attained a remarkable degree of mastery over his own mind. After a week trapped under the fallen structure, he ripped his way free of the rubble and emerged a new person - Mephiston, Lord of Death.
The astonishing increase in Mephiston's psychic powers led him to be named the Blood Angels Chief Librarian, the leader of the Librarius. Other Blood Angels hold him in awe. The fact that he conquered the Red Thirst proves that the illness is not invincible, and that gives the Chapter a new hope for salvation.
Mephiston in the game
Mephiston really earns his secondary title "Lord of Death" on the battlefield. He is one of the few beings extant in the 40K universe who can go toe-to-toe with a Tyranid Hunter-Killer and come out on top. Armed with a master-crafted plasma pistol and a force sword, he is more than a match for nearly any opponent in close combat. Chaos demons don't stand a chance against him. His only significant weakness is the possibility of being nailed down by long range heavy weapons fire early in a battle, so keep him under cover, keep him moving and he will wreak havoc in the ranks of the enemy.
Mephiston the model
The miniature model designed to represent Mephiston in a table-top battle is exquisite, and among the finest ever produced for Games Workshop. He sports a sinew and bone styled breastplate over a large flowing robe, with a massive cloak and large, ornate shoulder plates to top it off. There are a lot of skull and blood drop decorations thrown in for good measure. He is one fine looking fellow, and a pleasure to paint.