You acquire this for assassinating Tamir the Black Market Merchant in the town of Damascus.
This is the very first assassination of the game (Memory Block 2) and is very simple. After the cut scenes (dont forget the memory glitches) you will be in the middle of a market and Tamir will be going around to each stall and talking to the merchants. Simply walk up behind him and assassinate him with your hidden blade, simple as that.
Also don't forget about the memory glitches during the post assassination cut-scenes.