When Leonidus set foot on Nemea he was struck first by its similarity to his home of Macragge, the humans he found there reminiscent of the Illyrian tribesmen his father fought under Roboute Guilliman. Its soon became apparent that pacifying Nemea would be much harder than the esteemed 20th Chapter Master had anticipated. The humans lived in fear of the monster that roamed the harsh and broken landscape. The terrain limited the tactical scope of the Ultramarines to a gruelling ground war. The tribesmen at first feared the Space Marines, for there seemed little difference between them and the Giants, but Leonidus cemented his position and the reverence of the tribesmen when he killed the immortal and indestructible lion of Nemea, forever wearing its pelt as a sign of his prowess. Nemea was quickly brought into the growing realm of Ultramar, and its people recruited to the Legion, specifically to the 20th Chapter and there the axe was dissimilated throughout the chapter as the close combat weapon of choice. Leonidus himself was crafted a massive axe by Nemean axe-smiths, enhanced by the legions artificers and mechanium techpriests. In addition the Nemean Lion was displayed on many members of the Chapter. This practice was popular among recruits from Nemea but slowly grew out of favour as Nemea's culture changed and such practises were viewed as barbaric. Some traditions such as blood debts and duels of honour were deliberately retained by the Ultramarines. Nova City was established at the site of the death of the Lion and a monument was raised commemorating the confrontation. From a strongpoint of Nemea Leonidus led campaign after campaign. When Horus called the XIII and XVII legions together at Calth, Leonidus sent his 9th company, under Honorius Luciel, to act as ambassador to the Word Bearers. The fact that Luciel would be the first fatality of the Battle of Calth would have serious ramifications for the 20th Chapter and at the urging of Captain Corvo of the 201st company, and a native of Nemea, the 20th Chapter declared a blood debt must be paid and the Chapter painted their pauldrons crimson red. The furious Lions of Nemea hounded the Word Bearers, especially those under Dark Apostle Sorot Tchure to the very borders of the Maelstrom. The conditions of the Maelstrom made pursuit of the Traitors near impossible and so the Lions regrettably retreated to help bring the broken Imperium back together. When the Codex Astartes was placed down the 20th Chapter of the Ultramarines legion became known as the Nova Lions and the Fortress of the Lion was built on the borders of Nova City from parts of the Fortress of Hera. The ships of the 20th Chapters fleet were spread throughout the newly established Nemean Defence Force.
Sorot Tchure, however, would return to strike at the Nova lions, invading their empire and besieging Nemea itself, only 50 years after the Horus heresy had ended. Such and invasion could very well spell the doom of the young Chapter so Leonidus spread the majority of his chapter out to counter the attacks, while he led a small force to Deathfire Pass, accompanied by Captain Corvo. After a week of fighting the Nova Lions and their Auxilia were on the verge of being overrun so Leonidus sent Corvo to Nova City alone to join up with the reinforcements and lead them against the Word Bearers. Sorot Tchure was killed when he fought against Leonidus himself at the climax of the battle. The Nova Lions at Deathfire Pass were completely destroyed, their bodies desecrated. However the Death of Tchure fractured the Word Bearers, and left them easy prey for the Nova Lions and the Nemean Guardsmen. Corvo, now known as Corvo the One-Eye, was promoted to Chapter Master of the some 400 Nova Lions remaining. Though the Nova Lions had almost been wiped out, they would return to the galactic scene two centuries later, fully equipped and more wanting of vengeance than ever.