The last two seasons of 'Young Justice' were hailed for introducing some very obscure villains in the DC Universe, but those villains, as we find out in the Season 3, are just small fry compared to the villains to come.
We learn that the main baddies, aka the Reach and The Light, work on the base plans of Darkseid's earthly takeover, wherein they help convert humans into mind-controlled metahumans, through kidnappings and their combined technological prowess.
The unsung Leader of The Light, Vandal Savage, is all for the progress of humans into metahumans, but why would such an ancient human -- supposedly the first earthly metahuman being -- give up his home to a formidable conqueror like Darkseid? What could Darkseid possibly offer Vandal, now that Vandal already has control of Warworld?
Presumably, for the comics, Darkseid is more than likely caught in an on-going conflict with the people of New Genesis and his home planet Apokolips. Darkseid's forces mostly consist of throwaway hordes of Parademons (an artificially created servitor race who are loyal to their master, Darkseid), whereas New Genesis has a very large population of godlike metahumans. The people of New Genesis, The Forever People, quite often have the upper hand in the ever-unfolding war, and barely break a sweat in keeping the parademons at bay.
This results in Darkseid's army being heavily engaged in battle with The Forever People, and Darkseid has to send out agents like Desaad or Steppenwolf to infiltrate valuable planets quietly, in order to procure some advantage for Apokolips in their war.
Darkseid's need for the Light and the Reach are easily justified in this case, and Vandal and the Light offer very easy access to everything Darkseid might need for such a plan. They have a near unlimited source of finance (thanks to Vandal's vast fortune accrued over his 4000-year-old lifespan) and are able to influence politics on a global scale.
They've also already gathered most of the tech and resources that Darkseid would need for his operation. Without the Light, Darkseid would probably have to send his own people to infiltrate Earth's society, spend vast amounts of resources and countless hordes of parademons in order to put his plans in motion. With the Light, Darkseid would have most of his work already done for him.
The bigger question really is what does Darkseid have to offer Vandal?
Vandal and the Light all want to take over the world, remove the weak/non-metahumans, and basically set themselves up at the forefronts of a planetary dictatorship, with Earth being their base. But clearly, they think that they can't handle that job on their own, so they're relying on Darkseid for backup and tech to use against the Justice League.
The plan right now seems to be Darkseid helping the Light take control of the Earth, and the Light in turn revolutionizing the Earth, which can then be used as a factory to churn out Darkseid's metahuman army. But since there are supervillains involved, there's definitely going to a bit of backstabbing involved (I mean, they are supervillains after all).
After going through the arduous process of converting humans into metahumans, the Light might try to keep the spoils of hard work at bay from Darkseid in order to protect their planet's interests (the villains do live there too). Or we may find the powerful Darkseid just overrunning his supposed allies once the Justice League is weak enough from the Light's or the Reach's meddling.
The biggest question of the second half of season 3 is who will betray whom?
'Young Justice: Outsiders' is set to release the second half of season 3 in June, exclusively on www.dcuniverse.com.