Magus (魔王 "Maō"?, literally."demon king") is the tritagonist of Chrono Trigger and was the dark mage waging war against the Kingdom of Guardia in the year 600 A.D. Magus was originally Born as Janus (ジャキ Jaki?, "Jaki" in the Japanese version), the young prince of Zeal in the year 12,000 BC. He is especially determined to kill Lavos and find his older sister, Schala.
Both Magus and Janus look very quite different at first glance. Magus has pale gray skin, while Janus has skin typical of the other inhabitants of both the Kingdom of Zeal and average characters in other times. Both Magus and Janus have light blue hair; Janus, however, has shorter hair length than Magus. Janus also has rounded ears, while Magus has pointed ears.
Janus is about the same height as the various fiend imps shown in-game, while Magus is the tallest playable character. Janus also has very little muscle, while Magus has a rather muscular built.
Janus wears a light purple shawl cloak while wearing pointed boots of the same color. while Magus wore a cloak (blue in-game sprite, but red in the official artwork); dark magenta trousers; brown gloves, boots, a chest plate, and a codpiece, all of which appear to be made of leather.
As the Prophet, Magus wore a hood over his head with orange trimmings around the brim and collar.
Boss Battle Strategies
As a boss Battle, he possesses much higher HP and is generally way more powerful than any playable character, given the general level at which he is encountered. When he's recruited, his HP takes a massive drop and must relearn much of his former magical arsenal, though all of his other stats take a considerable boost. Given a sufficiently leveled PC Magus, he could defeat any of his boss counterparts with ease, so it is safe to assume that the drainage of power he took from Lavos was only temporary and he went on to become even more powerful than he was before.
Boss Battle at The Fiendlord's Keep
Magus is the first boss in the Fiendlord's Keep. He's easily the strongest boss faced at that point in the game. The battle with him happens in two phases.
In the first phase, Magus is protected by his powerful Barrier spell, which is a stronger form of the protection offered by the Barrier Sphere, and also has a very high physical defense. Each time he is hit, he will respond with Barrier Change, a spell that allows him to randomly shift to one of the four elemental weaknesses while absorbing the elements that are not weaknesses. In addition to the barrier-shift, he will cast a spell that matches his weakness; Fire II for Fire, Lightning II for Light, Ice II for Water, and Dark Bomb for Shadow. Out of all the Barrier shifts, the Dark Bomb is the only single-target spell. At some points in the first battle, he will cast Hell Geyser, less damaging than the barrier-shift spells, but it inflicts the Sap (HP Down) status. He will also use his physical attack, which does medium damage. Frog's Masamune is the only weapon that can inflict damage without causing Magus' magic barrier to change and lowers Magus' magic defense.
The second phase is marked by Magus dropping his barrier so that he can charge up for his ultimate attack. Crono and the team will have a few rounds to hit him with all the power they have. He will no longer counterattack during this phase. Before he casts his ultimate attack, he will cast a spell that does nothing to the party but signals that he is about to unleash the spell soon (the screen will read "Magus risks casting a spell!", followed by a circle of small green balls entering toward Magus). On the next turn, he will cast his ultimate spell: Dark Matter, which will inflict a lot of damage, enough to wipe out a character if not in full health (does about 230 HP damage if the party is at around Lv 22/23). The team has just a few rounds more to recover and do damage before he does it again.
Because of the need for diverse magic attacks in the first phase of the battle and high physical damage in the second phase, Robo is recommended as the third member of the group as Rocket Roll qualifies as a Shadow Magic attack for the first phase and Triple Raid is extremely powerful for both the first phase (when Magus is weak against Shadow) and the second phase. In addition, Frog has Heal and Robo has Heal Beam, which allows the two characters to heal the entire party alternatively in the first phase and to heal consecutively in the second phase.
Boss Battle at The North Cape
After the Blackbird quest, he awaits the party at the North Cape. When the party arrived, he first mocks Crono for wanting to assault Lavos at the Ocean Palace and then offers Frog - or the entire party if Frog is not present - an opportunity for revenge. If Frog or the party chooses not to fight Magus, he joins the party. If Frog or the party takes him up on the offer, the second boss fight against Magus takes place.
Because his power was drained away during the battle with Lavos, he can only use Lightning II, Fire II, and Ice II. His defense is no longer boosted and he no longer has the Barrier shift. even so, his magic attacks can still do a good amount of damage and hit all if unprepared.
If Frog is one of the three front-line characters and you choose to fight Magus, Frog will fight Magus alone. This match is much easier than having a party of three without Frog because players don't have to worry about massive healing as Magus' spells hit all every time. All that Frog needs to do is an attack with Masamune when HP is high and use healing techs when your HP is quite low.
As a Party Member
He can be recruited anytime after defeating the Blackbird. He is found in North Cape (meeting him is still an essential part of the story). After a short argument (in which Magus mocks Crono for trying to fight Lavos), Magus will ask Frog if he wishes to fight, or the whole party if they want to fight him if Frog isn't a member. If Frog refuses (if he does, his reason is that he knows killing Magus won't bring Crono or Cyrus back), Magus will join the party as they start to leave, not actually giving them a choice of letting him or not. Regardless of choice, he tells the heroes that there may be a way to revive Chrono with the aid of the Guru of Time. Canonically, Magus was spared and recruited, however, he was mentioned in Lucca's letter to Kid in Chrono Cross. This is supported by an extra shadow in Luccia's room while the letter is being read, although this may be a glitch of some kind.
In one of the New Game Plus endings (achieved if Lavos is defeated before fighting Magus), Frog is seen going to confront Magus, while he is preparing a ritual, the purpose of which remains unknown, although he states his fight with Frog will decide the fate of the world. The two then battle each other, the fighting happens off-screen. After that, a figure with a cape is seen standing on top of Fiendlord's castle, but it isn't known if it's him or Frog.