[Disclaimer: most of this information was taken from the Wiki articles, not directly from the comics themselves, and thus may be reflective of errors in the writeups.
But you have to admit, this is much more entertaining than the DC Wiki.]
Vonn: ” He was a prince of the Idyllists, a colony of Atlanteans that settled in the Hidden Valley 4,000 years ago. There, nestled in the capital city of Shayeris, the pacifist Idyllists became rabid builders and collectors, architects and artisans, and eschewed physical violence at all costs. The Idyllists were also practitioners of magic and kept an extensive library of mystical texts and scrolls. “
Vonn: we got some Hamlet/Lion King shit going down in the undersea
Vonn: You got two princely brothers — Thar and Zath. Thar becomes king, and he gets access to some magical whatnot
Vonn: Zath wants this shit, figures it’s “rightfully” his, and starts doing some fucked up necromancy and dark magic
Vonn: gets his dumb entitled dead-folk-messing-with ass kicked out of the Hidden Valley
Vonn: goes and fucks around with magic that shouldn’t be played with, becomes this monstrous dude known as Slizzath, because I guess that’s what you do when you ain’t got no herd of wildebeests to throw your brother under
Vonn: Thar’s all like “shit, my brother is seriously fucked up” and starts stockpiling weapons and robots to use against Slizzath
Vonn: but the other Idylists are like whoa back the fuck up on that King Thar, we are pacifists here in the Hidden Valley (the ranch dressing must remain conflict free)
Vonn: and then they…
Vonn: kill him?
Vonn: okay, yeah, I see how far your pacifism goes… just far enough to be fucking illogical.
Amy: You know, if the DC wiki did their writeups like this, I would read so fucking many more backstories.
Vonn: OH IT GETS EVEN COMPLEXIER
Vonn: Right, so before he gets kakked by the So Totally Pacifists Guys I Mean It! Idylists, King Thar jusssst manages to finish a magic spell to trap Slizzath in an otherwordly dimension
Vonn: BUT… this magic spell is linked to some mystical ritual which will give the offspring of the caster (Thar) soooooper cool magical powerssssss! /finger waggles
Vonn: so… the I’m Not Kidding We’re Really Pacifists Idylists exile Queen Berra (Thar’s pregnant wife) and send out the word to the rest of the undersea ALL babies born with purple eyes should be banished and killed
Vonn: knowing full well that it’s the Idylist mark of power and Queen Berra’s baby will in all probability be born with purple eyes
Vonn: jusssst so that none of these babies could possibly perform the magical ritual that will give them the magic powers and free Slizzath at the same time. I mean honestly what the fuck is that spell, cast one, conjour one free?
Vonn: I guess Slizzath’s undead army was also banished, so they’re worried about that too.
Vonn: and the Idylists also claim that King Thar went insane, in a bid to hide the reason behind their plan
Vonn: RATHER than be all “yo, one of our princes threw a bitchfit about not being king and started mucking around with dead people, so our brave heroic king did this wicked cool spell to seal him and his army away in another dimension. Like, totally DO NOT do this ritual thing because it will bring back our demented monster prince and his zombie horde, kthx.”
Vonn: Noooo we gotta lie about shit and ensure the death of a bunch of innocent babies
Vonn: that’s totally in line with our scholarly, mystical, and pacifist way of life.
Amy: Okay, so the thing I’m getting here is that Garth is the exiled rightful ruler of the Hidden Valley, which is currently being ruled by a ranch-for-brains council. He could probably ride back in with Atlantean backing and take it back, but unless he does that he probably doesn’t really have any royal standing outside of “Aquaman’s ward”?