Forbidden Blade is the second Super Rare weapon for the One Handed Stabbing weapon class.
Forbidden Blade has somewhat lower raw stats than Bloodletter. However, its special properties--its -0.1 initiative, 25% extra crit damage, and 5% faster cooldowns to all skills--can be just as good as Bloodletter, if not better, in terms of offense.
If using a hero such as Blackguard or Valkyrie with the intent of dealing damage, Forbidden Blade is preferred over Dark Defender which is more of a defense-oriented weapon. It remains a viable weapon until Piercing Blade comes into play, at which point the Forbidden Blade becomes obsolete in favor of Piercing Blade's higher level cap and especially the 20% armor piercing.