The way I see it, Lorgar has spent the last 10,000 years communing with the Gods and seeking true Enlightenment on the nature of Chaos. When he eventually returns, I have a feeling he will be the most powerful Daemon Primarch by then. Whatever he's doing on Sicarus, it's probably worthwhile.
I know people get down on Abaddon. But, Abaddon is the aliens in Xcom 2 in the Long War mod. Horus was the aliens in the original Xcom 2 campaign. One big battle. One decisive victory and the Imperium would have been screwed. Abaddon, on the other hand, is slowly gaining ground. There isn't ever going to be a major catastrophic failure for the Imperium. He's playing the long game. And it's working, and there's nothing the Imperium can do to stop it. If the black crusade succeeds, he wins. If it fails, he's still done irreparable damage that is slowly building up to be worse over time.
GW plot armor aside I'd have to sort of disagree on your "never going to be a catastrophic failure" statement. The Imperium is perfectly capable of catastrophic failure, but it is true that no single failure would lead to their fall. It's simply too vast an empire with too much war manufacturing capacity and too much manpower. If the Cadian Gate was smashed every world would be in peril, but that would be a slow process as Chaos can't reliably strike everywhere at once. Even if Terra itself fell it wouldn't be an I win button, it be a huge moral victory and their capacity to navigate the warp would be greatly diminished, but even broken and without leadership the lumbering leviathan would take a long long time to truly go down. However, besides these extreme failures there's still lesser catastrophes that can lead to the fall of sectors or intensify the threats to the Imperium. For example when Huron drew a force of Space Marines far from their fortress monastery and was able to commit a robbery of their geneseed. Such a catastrophic failure means the Corsairs will be a galactic threat for millennia. Or when production worlds fall to chaos insurrections. Some grander catastrophe is possible, and it could be almost entirely the fault of the Imperium, but yes, for the most part a Chaos Lord of any effectivenes needs a brilliant plan, a viscous campaign and the favor of the Fateweaver to chip away at the Imperium in any meaningful way.
Seeing your post I need to clearly clarify a bit more on the subject and you seemed to have missed a bit of information in my previous post, some of the black crusades were used to recover artifacts and the rest of them were just to build up energy for the warp for the final push to terra, that final push has yet to happen as your picture shows. Since the previous crusades had no other goal other then to recover artifacts and cause destruction to create energy it explains why none of the previous crusades went towards terra.
Three. The First, which by manner of it being so horrifyingly unexpected looked like it might succeed, and terrified the Imperium into taking the threat of the surviving Chaos legions more seriously, and bulking up their defences around the Eye. The Second, which was an attempt to do exactly the same thing again, only smack into the arms of the new Imperial defences and abject failure. (3 - 12 were then targeted towards different interim objectives, designed to either weaken the Imperium or strengthen Abaddon's hand, or both.) ...and the Thirteenth. Although a large chunk has become hideously bogged down around Cadia because the Imperium was wearing Matt Ward's plot armour and GW balked at actually committing to implementing the full result of the Eye of Terror live campaign into the lore, it's thought that this could be the one which finally punches through the Imperial defences and reaches Terra. Abaddon has sent one of his longest and most loyal companions to Terra in order to deliver a "Game Over" message to the Emperor ahead of the arrival of the legions. The Warmaster has recognised that the Golden Throne is failing, and if the Astronomican dies then it's curtains for the Imperium as although the Chaos fleets know how to navigate the warp without it, the Emperor's Finest haven't the faintest flying clue how to do it safely or reliably. That said, it's not as if Abaddon hasn't been crushingly wrong on this account every other time, so I'd advise against any Chaos players getting too excited about the possibility of actual genuine victory over the Imperium's adamantine plot armour dauntless defenders.
Iskandar Khayon - a former Thousand Sons sorceror who once tried to kill Ahriman, and the primary character in Talon of Horus, the first Black Library book to (shock! horror!) focus on the Black Legion. Took 'em long enough, but the book is great, so I've forgiven them. Khayon's explanation for handing himself into the Inquisition, and Abaddon's message to the Emperor is the final part: "I came because I am an emissary. I bring a message from my brother Abaddon, to be carried to the Emperor, before the Master of Mankind finally dies." I hear her breath catch in her throat. Instinct forces her reply before she can even consider what she's saying. "The God-Emperor cannot die." "Everything dies, Siroca. Even ideas. Even gods, and especially false gods. The Emperor is the memory of a man enthroned on a broken engine of false hope. The Golden Throne is failing. No one knows that better than those of us who dwell in the Eye. We can see the Astronomican dying. We can hear the Emperor's song fading away. I did not come to Terra to surrender myself into your hands in order to laugh at the dying of His light, but neither will I coat the truth in honeyed lies to make it easier for you to hear. "These are not reports on a screen to me, inquisitor, or reams of casualty figures to be easily discarded. The Emperor's Light is fading across the galaxy. How many fleets of vessels have been lost these last decades, to flickers in the Astronomican? Thousands? Tens of thousands? How many worlds have cried out in rebellion in the last ten years alone, or screamed in psychic distress? How many have fallen silent in the shroud of the warp, now home to nothing but the tread of daemons? Here, on Terra...can you hear any of Segmentum Pacificus' thousands of worlds? A quarter of the galaxy has fallen silent. Do you know why? Do you know what wars they are fighting, while cloaked in silence and shadow?" She is silent for a time. "What is the message you brought for the Emperor?" "It is simple enough. Ezekyle asked me to journey here and stand before our grandsire, just as we did when the Imperium was young. I will meet the dying Emperor's empty eye sockets and tell him that the war is almost over. At last, after ten thousand years of banishment in the underworld, his fallen angels are coming home. "These are the End Times, Siroca. None of you are destined to survive the coming of the Crimson Path. The Imperium has been losing the Long War since it was first declared, and now we enter the endgame."
