Ancient Rylanor

Rylanor was the Ancient of Rites of the Emperor's Children prior to the Horus Heresy. He was incarcerated in a dreadnought sarcophagus following a fight with the pointy ears people.

His loyalty to big E was so strong that not only did he nope'd his genefather's offer to join chaos (bear in mind that Fulgrim has enormous charisma that is jacked up on steroids by Daemonic shenanigans) but he even made Thousand Sons sorcerers pull the middle fingers on Fulgrim and let a virus bomb cook Istvaan III for a second time.

Yes, waiting 10 000 years just to shove it in Fulgrim's face was definitely worth it.