As with all the bosses in the Monastery of Ascension, Legio Quintus 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 Quintus 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 spawns "Lines of Ascension". These lines are spawned incrementally in the order of a horizontal line of ascension which bisects a the room through the tiles upon which the player stands (at 75% health remaining), followed by a vertical line spawned in the same manner (at 50% health remaining), and a final horizontal line spawned in the same manner (at 25% health remaining). Crossing the "Lines of Ascension" will damage players with an amount of damage similar in potency to being hit with his lightning attack. As with all other Legiones, his special ability's activation is signaled by an overhead chat message.
Legio Quintus is roughly Latin for "Fifth Legion". However Legio is a feminine noun, while Quintus is a masculine adjective; the correct form would be Legio Quinta. 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.