Talon of Horus

The Talon of Horus is the lightning claw formerly wielded by Horus, now wielded by Abaddon the Despoiler. After the Warmaster was killed in his confrontation with the Emperor, Abaddon took the weapon from his father's body and has used it since. Unlike most lightning claws, it has an integrated combi-bolter.

The weapon has an incredibly impressive kill count, including Sanguinius, Fabius Bile's clone of Horus and Sigismund. Not to mention Big E himself was injured by it. According to Iskandar Khayon, the psychic aura the weapon projects is overbearing, and any Blood Angel that confronts Abaddon goes completely berzerk due to it having been used to kill their Primarch.

Crunch-wise, it's a Sx2 AP-3 lightning claw with a combi-bolter attached in 40k. In 30k it's an AP2 lightning claw that debuffs anyone wounded but no killed by it with -1WS and -1S, allowing Horus to win every duel he gets into.

The Novel

It's also a book by Aaron Dembski-Bowden about the founding of the Black Legion, and part 1 of his rehabilitation of Abaddon's reputation. The novel follows Iskandar Khayon, a former Thousand Sons sorcerer, and his involvement in the hunt for Abaddon and the Vengeful Spirit.

What's awesome

• A good part of the story is a somewhat longer What it's like, about how politics, power and daily life for a Chaos Marine works in the Eye of Terror.
• The protagonist and his confederates aren't portrayed as one-dimensional cartoon villains, but as calculating demi-gods who have their own reasons for wanting the Legion Wars to end.
• Sheds a good light on Abaddon, making him seem like a man who warriors would want to follow. Either he's much less arrogant than he was portrayed during the Horus Heresy novels, or he was putting on an act for Khayon and co. Either way, shows he's at least somewhat capable.
• Features a good example of a (relatively) reasonable Khorne devotee: Lheor.

What's not awesome

• Bit of skub, people think ADB is a Chaos Fanboy because he happens to write good books where Chaos wins. It should be pointed that he wrote other books like Helsreach, Ragnar Blackmane and Emperor's Gift, which shows the Imperium kickin ass.
• There's a fair amount of skub around Khayon's two companions: a Dark Eldar Scourge/Incubi who Khayon is keeping alive with psychic powers, and a benign daemon or warp-creature that acts as his familiar.
• Seeing as the entirety of the book is dictated to the Inquisition while the narrator is in custody, many of the things he says are suspect (such as his claim that Chaos is fated to eventually overthrow the Imperium).
• The main character is a bullshit powerful Mary Sue, so much so there's an entire section about it on his page.