Warning: SPOILERS for Supergirl season 5, episode 8, "The Wrath of Rama Khan."
Supergirl's midseason finale revealed that the Monitor had one more hero to test in building his army to face the Crisis on Infinite Earths: Martian Manhunter. The Martian Manhunter J'onn J'onzz passed the test with flying colors, proving his worth to stand among the Worlds' Finest heroes.
The Monitor first appeared during the 2018 Arrowverse crossover event Elseworlds, testing the heroes of different alternate Earths and seemingly destroying those who failed his test. As the heroes of Earth-1 and Earth-38 joined forces to combat Dr. John Deegan, whom the Monitor had empowered with reality-altering Book of Destiny, they discovered that the Monitor was not testing the heroes' powers but their moral fiber. This led Oliver Queen to challenge the Monitor over the morality of a test that was guaranteed to kill The Flash and Supergirl and make a bargain, promising his help in battling the coming Crisis and aiding the Monitor in gathering the army and weapons he needed to save all reality.
Supergirl's season 4 finale revealed that the Monitor was not yet done testing the heroes of Earth-38, with one of the episode's final scenes showing the Monitor freeing a Green Martian from some manner of celestial prison and telling him that he would have the chance to avenge himself upon the brother who wronged him. This Green Martian was later identified as Ma'alefa'ak J'onzz; younger brother of J'onn J'onzz. Throughout the first half of Supergirl season 5, J'onn was forced to deal with his long-lost brother, and in doing so, he proved to the Monitor in the episode "The Wrath of Rama Khan" that he was worthy of fighting alongside the rest of the multiverse's heroes in taking on the Anti-Monitor.
The events of Supergirl season 5 revealed that Ma'alefa'ak had been born with an inability to connect to the Green Martian hive-mind and the power to force his will onto others, telepathically. Ma'alefa'ak and J'onn's father, M'yrnn, reacted with fear, electing to seal his son away and pray that the Martian gods might heal him. When this only isolated Ma'alefa'ak further and caused him to join the war-like White Martians during the Martian civil war, J'onn sought to ease his father's shame at what his actions had wrought by erasing all memory of Ma'alefa'ak's existence from the Green Martian hive-mind. This was the greatest taboo in Martian culture and the Martian Manhunter's secret shame.
Forced to face what he had done in trying to ease his father's conscience, J'onn found himself tested as never before over the course of Supergirl season 5. In the end, faced with the choice of either banishing his brother forever to the Phantom Zone or risking his psychic essence by opening himself up to Ma'alefa'ak and melding their minds so that he feel J'onn's honest remorse for his actions, J'onn chose to surrender himself to his brother. The gambit paid off and the two brothers reconciled as they realized the harm they had caused one another over the years.
Supergirl season 5 midseason finale, "The Wrath of Rama-Khan", ended with Ma'alefa'ak using his powers to help save Earth from Lena Luthor's Project Non Nocere and a grateful J'onn giving his brother his shape-shifting Martian spaceship, so that he could return to Mars and join the resistance there. As J'onn watched Ma'alefa'ak fly away, the Monitor appeared and congratulated J'onn on passing his test, saying that he had sensed that J'onn was "a great warrior, but one plagued by his past." The Monitor added that he had hoped J'onn would make peace with his brother and that he was satisfied that J'onn was now ready to face the coming Crisis.