Silixiz's Tower Key

Source
This item can be pickpocketed from Foreman Silixiz outside the tower north of Sludge Fen in the northeastern corner of the Barrens.

When you defeat the boss of the quest, at the top of the tower, your player gets poisoned. The poison is special, last 7 days and prevents you from using stealth.

The poison is removed when you complete the next quest in the chain, or if you have a high(?) level player cast some form of remove poison spell.