As with all the bosses in the Monastery of Ascension, Legio Tertius attacks primarily with a combination of a basic magic attack and a lightning attack - the latter of which is focused in a 2x2 square (expanding to a 3x3 square and then to a 4x4 square over two game ticks) which deals high, magic-style damage to the player if they stand where the lightning falls. (The lightning attack is always affixed to the player's location, and thus continuously moving throughout the fight is imperative.)
Like all of the Legiones within the Monastery of Ascension, Legio Tertius has a special ability which activates as his life points are gradually depleted in 25% intervals (every 7,500 life points drained) until death - occurring a total of three times throughout the fight. His special ability, however, is that he summons other ascended to aid him throughout the fight. As with all other Legiones, his special ability's activation is signaled by an overhead chat message. In total, he summons three Rorarii at 75% life points, one Gladius at 50% life points, and finally a Capsarius at 25% life points remaining. Once the capsarius uses it healing buff on Tertius, it will need to be killed - though it is possible to kill Tertius before the capsarius focuses on him, using a combination of high-level weapons and powerful, well-timed abilities. The ascended he summons do not drop anything. When Tertius dies, all his summoned ascended also die.
Legio Tertius is roughly Latin for "Third Legion". However Legio is a feminine noun, while Tertius is a masculine adjective; the correct form would be Legio Tertia. Legions were the largest units of the Roman army and there were 6 in a whole army, hence there are six legiones in the Monastery.
Legio Tertius' summoned ascended did not die with him prior to an update on 29 June2015 - players had to kill them after he died, or leave the room and create a new instance.