Note: this article contains "spoilers" to the Uriel Ventris novels. Read at your own risk.
Born in 876.M41 to the same family as 1st Company Veteran Sergeant Lucian Ventris (who died on Macragge's northern Polar Defenses during the Battle for Macragge), Uriel Ventris was recruited by the Ultramarines in his 6th year. Uriel trained at the Agiselus Barracks, where he graduated in 889.M41 to train at the Fortress of Hera. Nine years later, he became a full-fledged Marine in the Scout Company, and was finally inducted into the Fourth Company in 909.M41.
History as Captain
At the Battle of Bridge Two-Four on Thracia in 999.M41, Captain Idaeus of the Fourth Company fell, leaving Ventris in battlefield command. Shortly after, he was confirmed as the new Captain of Fourth Company. Ventris' first mission as captain was to put down a rebellion on the Imperial world Pavonis, where Ultramarines encountered the Nightbringer, newly awakened from his millennia-long slumber.
Ventris and the Fourth Company was called out once more in 999.M41 to repel a Tyranid threat from the planet Tarsis Ultra, with the assistance of an Ultramarines descendent Chapter, the Mortifactors. However, Ventris' tactics in eliminating the Tyranids were unorthodox in the extreme, and as punishment, Chapter Master Marneus Calgar exiled both Uriel and his companion Pasanius and bound them to a death oath.
Taken captive by a daemon known as the Omphalos Daemonium, Uriel and Pasanius found themselves on the Iron Warriors homeworld, Medrengard. There, they did battle against the forces of Chaos as best they could, and finally managed to escape on the Omphalos' craft.