The Primarch's has no name or titles.
After being scattered through the warp, the primarch landed on a Daemon World. Miraculously, he was never corrupted by Chaos, and displays an extreme hatred towards daemons.
Upon discovery by the Emperor, the Second Legion was put under the primarch's command. The legion was give the name "Doom Slayers", as testament to their resilience to chaotic corruption and effectiveness in fighting daemons.
The Doom Slayers never took part in the Great Crusade, much to the Emperor's disappointment. Instead they immediately returned to the Eye of Terror, following their primarch in his neverending crusade against chaos, never to be seen or heard from again.
Records of the Second Legion were expunged from Imperial records.
Periodically reports come from worlds who have experienced daemonic invasions telling of a unknown chapter of Space Marines intervening in the battles. Clad in green power armour of heresy-era type, these Marines were at first assumed to be a Chaos Space Marine warband.
Further information came to light showing these marines fighting daemons, and no chaotic corruptions could be seen on their wargear. They specialize in close-quarters combat, with a heavy focus on shotguns, hand-to-hand combat and chainsword use, preferring to tear daemons limb from limb to kill them.
Grey Knight detachments have noted that these marines are willing to aid those fighting daemons, although they will disappear into the warp after the daemons who attempt to retreat.