The Grand Maester, First of the Order, was the head of the Templar Order.
The maester believed that humans were flawed beings, and the only way to make them immune to temptation was to cleanse their minds, making the recruits impervious to both the corruption of demons and the blind righteousness of angels.
The maester was revered in the Order. He slew innumerable evils, as well as some innocents. He was the one who perfected the Order's purification procedure and sanctioned the actions of the Templar Inquisitors.
When Kormac returned to the Order in Westmarch, the Grand Maester said that he wanted him to join the order again, and eventually become its leader himself. Kormac refused, and the Grand Maester ordered his Inquisitors to kill him, only to be slain in the battle that followed.
The Grand Maester is a unique Inquisitor, encountered during The Templar's Reckoning follower quest in the Templar Outpost. In combat, he has Reflects Damage and Extra Health affixes, and also possesses all skills of Kormac, including the ability to heal himself and his allies. To finish the quest, both the Grand Maester and all his guards must be killed.
Note that the Grand Maester is fought at two locations: once he loses enough Life, he will flee from the first hall, awaiting Kormac and the Nephalem with reinforcements in the second room.
Upon death, he drops the Grand Maester's Journal.
The Grand Maester is armed with the Heraldic Heater shield and a javelin. However, in patch 2.1, the legendary spear Akanesh, the Herald of Righteousness was added, explicitly stated to have been owned by the Grand Maester before. It did not, however, replace Grand Maester's in-game weapon.