Eldrad Ulthran

Eldrad Ulthuan is the Eldar Farseer of Craftworld Ulthwe. He is a pivotal character whose mysterious activities have helped shape the history of the galaxy, as he uses his foresight to try and prevent Chaos from overtaking realspace. Despite this, he has a rather infamous reputation as his plans usually end up with someone getting killed or causing massive collateral damage.

Official Canon
Eldrad was involved in the Horus Heresy, when he tried to warn Fulgrim of the impending threat of Chaos, not knowing that the Primarch had already fallen to its clutches. He also interfered with the Cabal’s plans of turning Alpharius Omegon into their pawns and aided Vulkan to reach Terra at a time when everyone had believed him to be dead.

In the following years he was focused on saving other Craftworlds from Xenos attacks, such as by diverting Ghazghkull Mag Uruk Thraka’s Forces to Armageddon.

In Gathering Storm, Eldrad and a team of Harlequins performed a ritual to summon the nascent Eldar god Ynnead. Though the new god wasn’t at its full power, it was enough that it could help found a new faction of Eldar called the Ynnari. Because of the ritual, Eldrad has been exiled from Ulthwe, forcing him to join the Ynnari.

TTS Canon
Having foreseen that Abaddon the Despoiler’s Thirteenth Black Crusade spelled trouble for the Eldar, Eldrad contacted Asdrubael Vect to give his warnings, and despite how much he loathed him, asked for his help. Eldrad intended to perform a ritual to summon the Eldar death god Ynnead, and he needed Vect to distract the Imperium with a raid nearby. Vect refused, but Eldrad decided to go ahead with the plan, with the reassurance from Inriam's Spectre and Sylandri Veilwalker that they would help him find Ynnead’s herald. Meanwhile, he said to contact Cypher to go forward with his plans on Mars.

Trivia
Eldrad is canonically a crystal meth addict in canon.