While trapped in the Diamond Realm, once the protagonist fetches two Somas for En no Ozuno he will then have him fight the Red Phantom and the Blue Phantom (though one of the battles can be skipped if the protagonist's alignment matches either of theirs) to gain alignment-exclusive weapons (the Light Sword for Law and the Headsman Ax for Chaos).
The elementals of the magic spell they use are correspondent to the Law Hero (force) and Chaos Hero (fire).
Strangely, the Phantoms use the color representative of the opposite alignment to theirs (when encountering the Blue Phantom, the Chaos theme plays, while facing the Red Phantom, the Law theme plays).
This is swapped in the Sega CD version. Furthermore, in this version the portraits of the phantoms shows the future Law Hero and Chaos Hero's faces. Messiah Law Hero for the Blue Phantom and demonized Chaos Hero for the Red Phantom.