I'll try to give you a short summary of it all.>1st
Cemented the dominance of the Black Legion and his rule over it. He also acquired Drach'nyen from the Tower of Silence on Uralana, while Imperial forces were held up by the assault on Cadia.>2nd
Destroyed an Imperial Navy base and the shipyards on Belis Corona. Also cursed the planet's moon to unleash a mutagenic plague upon the planet's inhabitants in the future. Later sacked the Inquisitorial vaults on Nemesis Tessera – butchering the Imperials and freeing all the Daemons imprisoned in the fortress.>3rd
Unleashed the Daemon Prince Tallomin on the Cadian Gate. Tallomin is banished by a dozen Chapters of Astartes, after slaughtering many millions of Imperial soldiers. It was all a distraction, as Abaddon could now desecrate the tomb of Saint Gersthal on the shrine world of the same name, thereby preventing a prophecy of ultimate Imperial victory from coming true.>4th
Assaulted the world of El'Phanor, a stronghold of the Cadian Gate. The planet was left a dead wasteland, leaving Cadia with 'a chink in its armor'.>5th
After provoking a massive Imperial retaliation, Abaddon makes a sacrifice of ten million Imperial citizens to summon Doombreed. Two Chapters are killed in the trap sprung in the Imperial city of Kasyr Lutien, the Space Marines' violated by Daemons and their gene-seed forever lost to the Imperium.>6th
Abaddon allied with Drekarth of the Sons of the Eye, a faction that defected from the Sons of Horus when Abaddon assumed leadership of the Legion. Together they annihilate a Forge World. Abaddon executes Drekarth, and the dying warlord is forced to watch as his men pledge allegiance to Abaddon.