Sir Zeliek

Sir Zeliek was one of the Four Horsemen of Naxxramas. He was a paladin in life, so strong in his faith, that even in undeath the power of the Light still heeded his call, smiting his foes in battle. Though his body was forced to obey Kel'Thuzad's commands, Sir Zeliek managed to retain his sanity and morals despite his corruption. This makes him a tragic character, forced to slay what he once held dear in his heart against his will.

Ashbringer
Sir Zeliek witnessed the enslaving of Alexandros Mograine by Kel'Thuzad. He then fought against the party Darion Mograine led into Naxxramas. He stabbed Brannigan Thundermaul in the back, asking for forgiveness as he did so. Before he died, Brannigan managed to strike Zeliek in the head, temporarily taking him out of the fight. When Zeliek stirred, the battle was over, and he was left alone to lament his cursed existence.

Wrath of the Lich King
Sir Zeliek survived his encounter in the first siege against Naxxramas in the Eastern Plaguelands, and returned as one of the Four Horsemen in the newly relocated necropolis, floating above the Dragonblight.

Ashbringer

 * Leave, I beg you, while you still can!
 * You know that I take no pleasure in this, Korth'azz.
 * Forgive me, brother. This is not my choice. But you must die. The master commands it!
 * When will it be over? When will I be released? How many more must I kill?

Naxxramas

 * Intro
 * yell
 * yell
 * yell
 * yell
 * yell
 * yell
 * yell
 * yell


 * Taunt
 * yell
 * yell


 * Aggro
 * say


 * Killing a player
 * say


 * Death
 * say

Trivia

 * Zeliek is voiced by Dave Kosak.
 * An unused NPC representing a modern version of Sir Zeliek was added roughly at the same time as matching versions of the three other original Horsemen, all presumably meant to appear in World of Warcraft: Legion. He and these other NPCs didn't make it out of the Legion alpha stage. See Original intent in Legion.
 * He's possibly analogous to the Horseman of Conquest, one of two most popular interpretations of the 'White Horse', who's depicted as a glamorous and regal figure in shining armor.

Speculation
It is possible that Zeliek didn't meet his final death since he already returned once, just like Baron Rivendare.