Cloak of the Thief
|A pitch-black cloak which allows its wearer to excel in the arts of thievery, and can be evoked to release thick clouds of smoke for a hasty escape.|
Being able to generate clouds of smoke and fog on demand is incredibly powerful when used correctly - it allows you to ignore all threats that are not within a 2 tile radius of you, almost like a better version of the Darkness spell. This is really strong for melee builds and most definitely worth the -2 slaying. Even for characters with ranged weapons, the ability to block line of sight to dangerous ranged enemies like fire giants or golden dragons makes this cloak useful.
Evoking the cloak of the Thief is as difficult as evoking an amulet of rage -- at 0 Evocations, it has a 50% chance of working, which can be manageable as long as you don't mind losing a couple of turns trying to set up a smoke cloud. Improving your Evocations skill improves your chances of success (24% failure rate at 6 Evocations, 10% at 11 Evocations); around 6-8 ranks in Evocations is probably plenty if you don't have any other reason to invest in evocables.
Prior to 0.15, this cloak only penalized damage (-3), not accuracy, and provided a slightly higher stealth bonus.