As is mentioned, this is dropped by the Itraer Vius, a raid mob in Akheva Ruins.

there's also a pal/shd 1hb rancorous skull of whatever that drops from the cursed cycle in ssra, 21/21 15 int and 100hp/mana i believe its a really nice weapon, go get it omg

Ancient prismatic mace as well.

Skybreaker from Va'Dyn in Akheva also looks like a skulled club with spikes.

The weapon shows a damage bonus because all weapons wielded by a 51+ character (a character that has a damage bonus to begin with) show one. Grab a bow and look at the damage bonus on that. However, the damage bonus is applicable ONLY to primary hand weapons (including 2-handers, since they're equipped in the primary slot). Neither secondary nor range weapons apply damage bonuses. For example, my Swiftwind, Defiance, and Bow of the Huntsman all show damage bonuses of 9. I've hit for 1 damage with my bow and Swiftwind, while the minimum damage on my Defiance is 10 (1 damage + 9 bonus).

To better compare offhand and mainhand, I'll use the example of my Swiftwind and a Frostbringer. I know the FB is one less damage, but I can speak from experience about that, not theoretically about a 13 damage mainhand weapon. Max damage (using self-buffs only so as not to skew results) with my Swiftwind is 63. Max damage with a Frostbringer was 67 (though I only used the FB for about a half hour). If the bonus was applied to the offhand weapon, the Swiftwind would have maxed out at around 69-72 or so.

Edited, Mon Feb 11 03:59:35 2002

The graphic on this is wicked...I alternate between this and my tri-plated golden hackle hammer in secondary slot...TPGH looks like a CHoS, this on the otherhand is rare to see someone with. Ok stats, I have yet to aug it though.

Brother Uncontrolable <Truly Naughty Troop>
65th Trancendent of Torv