GKTiberius wrote:
by the very nature of being a traitor, outlaw, and chaos warlord, that fact that he is still around to do allo f those thing after 10000 years is a legacy unto itself. Him, Ahriman, Typhus, and to a lesser extent Lucious and kharn are on equal footing with him and this point, despite that abaddon has a larger following than the others. I think by Legacy you mean Impact on the galaxy. from a certain point yes, because he has done more than horus or any other primarchs by simple factors of time, but I would say from another point of view no because the magnitude of the events of the heresy and acts of the primarchs over shadow all the events of the subsequent 10000 years.

but the question is kind of moot. as Abbadon wouldn't be able to do what he is doing without horus or the other primarchs,

 Marshal Loss wrote:

 DarknessEternal wrote:
In 200 years, Horus conquered the known galaxy. In 10,000 years, Abaddon hasn't even managed to fully open the door of his prison.
After that, he went traitor and didn't do as much conquering.

Read HH: Conquest! He no longer sits on his throne twiddling his thumbs for most of the Heresy.

Anyway, Abaddon's legacy ultimately depends on whether he succeeds or not. He's a different man in a different era, aiming to succeed where Horus failed in a vastly different time period. Comparisons are inevitable by virtue of Abaddon's past but they're pretty pointless, especially while we're still waiting for ADB to pump out a few more novels in the Black Legion series to flesh out his achievements/failures.

EDIT: ADB also pumped out a fantastic blog post on Abaddon

 ferret61 wrote:
Abaddon>Horus as everything Horus has done is fictional.
Whereas Abaddon has a real legacy, his arms will forever be down the back of my couch.