Warning! SPOILERS for Young Justice season 4 finale.
The finale of Young Justice season 4 featured a scene after the closing credits which explained the fate of the missing Mary Marvel. It also introduced the Earth-16 version of Kara Zor-El, whose arrival in the world of Young Justice has been highly anticipated since the series' beginning. The twist, however, is that these two young heroes have been brought into the service of the New God tyrant Darkseid and stand poised to attack Earth in Young Justice season 5.
The Young Justice season 4 finale was largely focused on the heroes of Earth holding back an invasion of rogue Kryptonians led by General Zod and his son, Lor-Zod, but the episode also offered an update on the activities of Vandal Savage and his allies in the villainous organization known as the Light. Savage was revealed to have captured all of the villains the heroes had sent back to the Phantom Zone. All of them were contained within his base on the artificial planet Warworld, save for one unspecified Kryptonian captive and the White Martian terrorist Ma’alefa’ak, who had been sent to the planet Apokolips as tribute to the evil New Gods.
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The identity of this mysterious Kryptonian was confirmed in the post-credits scene of Young Justice's season 4 finale, as Darkseid's son Grayven presented the corrupted Mary Marvel and Kara Zor-El of Krypton to his father. Introduced as the latest recruits for Granny Goodness' elite team of assassins, the Female Furies, Mary and Kara were identified as Black Mary and the Supergirl respectively, with Grayven speaking as to both young women being "ready to take the battle to the so-called heroes who betrayed
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