|Base item||long sword|
|Damage vs. small||1d8 +1 + level drain + cancellation + beheading|
|Damage vs. large||1d12 +1 + level drain + cancellation + beheading|
|Weapon skill||long sword|
|Base price||1500 zm|
Thiefbane is always generated in the inventory of One-Eyed Sam (unless wished for before Sam is generated). Thiefbane is typically generated with an enchantment, but the greater prize is the instadeath by decapitation (10% chance) on monsters symbolized by @. Thiefbane also has a 1 in 6 chance of canceling whatever it hits, which is probably you.
Note that Thiefbane may blast you, even if you are chaotic.
Given that One-eyed Sam is level 24 and carries most of an ascension kit, it's usually a bad idea to simply engage them in melee. A scroll of earth or invisibility will fix One-eyed Sam on their spot, and highly enchanted firearms or poisoned darts work fine for the kill. More suggestions can be found at the black market page.
However, this is not neccessary to acquire Thiefbane. If you are appropriately fast, armored, brave or stupid, and have the disarm technique and are wielding something you are at least skilled in, you can steal Thiefbane. Be aware that you still risk decapitation. If you try that, you will need an escape plan that works despite the black market's restrictions. One way is to apply (break) a wand of digging, trapping him (away from the door). Note that unless you are wearing an amulet of life saving, you don't really need an escape plan, since you won't be around to use it.
As if that's not hard enough, UnNethack does not allow Thiefbane to be wished for (the code calls this
wishing abuse). So you're going to have to fight her and her sword.