Abaddon doesn't ignore invulnerable save any more, as far as mastery...
The book implies (since it states that it is) that it is a daemon weapon, all be it and very unique one, but just a daemon weapon so I would assume he doesn't have any kind of mastery over it (it would/should state if he did). it clearly states that he roles just like anyone else to determine how many attacks it gives him so... don't roll a one |