There are thirteen secret achievements:

Visions of the Future (50 points): A strange vision has appeared to you. 
What
could it mean?

The Blood of a Slave Trader (25 points): Slay Talal, Slave Trader of 
Jerusalem.
The Blood of a Doctor (25 points): Slay Garnier de Naplouse, Hospitlier 
Leader in Acre.
The Blood of a Regent (25 points): Slay Majd Addin, Regent of Jerusalem.
The Blood of the Merchant King (25 points): Slay Abul Nuqoud, Merchant King 
of Damascus.
The Blood of a Liege-Lord (25 points): Slay William of Montferrat, 
Liege-Lord of Acre.
The Blood of a Nemesis (25 points): Slay Robert de Sable, but there is one 
more.
The Blood of a Teutonic Leader (25 points): Slay Sibrand, the Teutonic 
Leader of Acre.
The Blood of a Scribe (25 points): Slay Jubair, the Scribe of Damascus.
Welcome to the Animus (20 points): Successfully complete the Animus 
tutorial.
Hero of Masyaf (20 points): Protect Masyaf from the Templar invasion.
The Punishment for Treason (20 points): Find the traitor and bring him 
before Al Mualim.