Shiv

A shiv is a large needle or nail used as a knife-like (stabbing) weapon. It's about 3 to 5 inches in length with cloth wrapped around the rear as a handle.

Location
It can be acquired in the quest No One Escapes Cidhna Mine. Borkul the Beast asks the Dragonborn to find a shiv for him before he allows them entry to see Madanach.

Grisvar the Unlucky agrees to trade a shiv in exchange for Skooma which Daunch can be persuaded to hand over for free. Alternatively, access to Madanach's cell can be achieved by simply giving the Skooma to Borkul or by selecting the dialogue option Brawl.

The alternative to obtaining the shiv is to do the wooden plate glitch through the locked doors until you reach any npc.

Additionally, Lisbet at Arnleif And Sons Trading Company sometimes sells shivs. However, the Dragonborn is unable to smuggle them into Cidhna Mine for quest purposes.

Quests

 * No One Escapes Cidhna Mine