As with all the bosses in the Monastery of Ascension, Legio Primus 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 Primus 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 both his basic magic attack and lightning attack will increase in potency as his health is depleted incrementally - much like the special ability of Dharok the Wretched, only capped in 7,500-life point intervals. As with all other Legiones, his special ability's activation is signaled by an overhead chat message.
Legio Primus is roughly Latin for "First Legion". However Legio is a feminine noun, while Primus is a masculine adjective; the correct form would be Legio Prima. 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.
A few days after release, the Legio's drop table was updated; drops such as bowstrings were replaced with more valuable items such as the magic shieldbow (u).