Basilisk

Basilisk (unknown - 1190) was the leader of the Templar Order until his death in 1190.

Biography
Upon traveling to Jerusalem, the Assassin Altaïr Ibn-La'Ahad learned that Basilisk was the Templar commander in the region, and that he held one of the three keys to the Temple of the Sand. Believing the Chalice to be held within the Temple, the Assassins infiltrated a party, hosted by the King of Jerusalem, where they knew they would find Basilisk.

Altaïr and Basilisk faced off against each other, and the Assassin eventually managed to best the Templar leader. While Basilisk managed to escape soon after, his key was lost to the Assassins.

With all three keys now in his possession, Altaïr traveled to the Temple of the Sand and managed to infiltrate the room where the Chalice was kept. However, he only found Lord Basilisk standing besides an empty chest. Basilisk started taunting the Assassin, also hinting that the Chalice was not an object but a woman. He then left the Assassin within the Temple, which soon started collapsing.

The next time the two men met was in the Templar stronghold in Tyre, where Altaïr and Basilisk faced off once more; the Assassin managed to wound Basilisk badly. The Templar leader was forced to reveal that the Chalice was in fact in Jerusalem, and that his forces were besieging Acre. The Assassin then burned all of his ships, delaying Basilisk's journey to Jerusalem.

However, Basilisk traveled straight to Alep, where he managed to take Adha, the Chalice, while Altaïr was distracted by Harash. Basilisk took Adha to the Templar port in Tyre, where Altaïr soon confronted him. This time, the Assassin managed to kill Basilisk, though Adha turned out to be on a different ship, which had already left port.

Upon Basilisk's death, it is assumed he was succeeded by Robert de Sable as Templar Grandmaster sometime before the events of Assassin's Creed

Reference

 * Assassin's Creed: Altaïr's Chronicles