Ragnarok and Odyssey

Ragnarok and Odyssey are among the strongest and most popular Psynergies in Golden Sun. Both are single-target Venus attacks that can only be wielded by Isaac and Felix when they are in the Squire class series (only Venus Djinn are equipped). The series - or, at least, Ragnarok - appears to return in Golden Sun: Dark Dawn based on an E3 2010 floor demo.

Ragnarok
Ragnarok is available at Lv. 13 to the Squire, Knight, and Gallant classes (the Lord class also has it in the first Golden Sun). It is described as a "strike with a massive sword", and is the most powerful of the original single-target Psynergies. Ragnarok is often obtained just prior to the first battle with Saturos, and as such, it is considered extremely helpful in the battle. It costs 7 PP to use and strikes the target with an attack equal to the user's standard physical attack, with 35 additional points of damage added and then the resulting damage either increased further or decreased depending on how the user's Venus power compares to the target's Venus resistance.

Odyssey
In Golden Sun: The Lost Age, after reaching the Lord Class, Ragnarok is replaced completely by the far more powerful (and expensive) Odyssey. Odyssey, which is described as "pierc[ing] a foe with a colossal sword", appears as the caster piercing his target with two small, rapier-esque blades before slamming into them with the aforementioned "colossal sword". Because it can be obtained before the player fights Poseidon, and thus is usable for most of all of the middle and late game, Odyssey is often used in favor of Ragnarok, although some players think its high cost of PP, when compared to the relatively low amount of PP that Isaac and Felix possess, make it not worth using. Odyssey costs 18 PP to use and strikes the target with an attack equal to the user's standard physical attack, with 95 additional points of damage added and then the resulting damage either increased further or decreased depending on how the user's Venus power compares to the target's Venus resistance.

Golden Sun: Dark Dawn
Ragnarok was seen in Matthew's list of usable Psynergy at 2010 E3 Battle Mode. It costs 7 PP to use, is Venus aligned and strikes one target, and therefore has most likely remained unchanged.

Cultural references
Ragnarok is the name given in Norse myth to the battle that will ultimately cause the end of the current world and the creation of the new. The Psynergy was likely given this name simply to exemplify its violent nature.

Odyssey is the name of an epic poem by Homer, deriving its name from its hero, Odysseus (Ulysses in Roman sources).