As with all the bosses in the Monastery of Ascension, Legio Sextus 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 Sextus 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 will spawn lightning until three bolts of lightning persist simultaneously within his dungeon. The lightning which he spawns persists longer than those spawned by other Legiones, the length of time which each bolt of lightning persists increasing incrementally as his health is depleted. Once a total of three bolts of lightning have been spawned (within the first few seconds of the fight), the eldest of them will disappear and respawn at the players' location, generating a cycle of three bolts of lightning which will persist until Legio Sextus is defeated. As with all other Legiones, his special ability's activation is signaled by an overhead chat message.
Legio Sextus is roughly Latin for "Sixth Legion". However Legio is a feminine noun, while Sextus is a masculine adjective; the correct form would be Legio Sexta. 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.
Although Legio Sextus uses his lightning attack throughout the whole kill, he will occasionally retaliate to an attacking familiar with a basic magic attack.