Gerard de Ridefort

Gerard de Ridefort (c. 1140 –  4 octobre 1189) was a Flemish crusader and a Grand Master of the Order of the Knights Templar.

Supporting Guy of Lusignan side against Raymond III of Tripoli, his own personal ennemy, the Grand Master contributed to the nenewal of the war between Saladin and the Kingdom of Jerusalem. Having provided pretexts for the war, the Grand Master pushed the new King, Guy de Lusignan, in the Battle of Hattin, which was won by the muslims and sealed the fall of the christian kingdom of Jerusalem.

Reference

 * Assassin's Creed Encyclopedia