In this chapter of the Udyoga Parva of the Mahabharata, Arjuna delivers a message to Duryodhana, urging him to reach a peaceful resolution to avert the impending war. Arjuna says that if they go to war, the Pandavas will surely win because they have Krishna on their side. He recounts the wondrous deeds of Krishna:
[T]hat Vasudeva, viz., who having mowed down in battle by main force all the royal warriors of the Bhoja race, had carried off on a single car Rukmini of great fame for making her his wife; and by her was afterwards born Pradyumna of high soul. It was this favourite of the gods, who, having speedily smashed the Gandharas and conquered all the sons of Nagnajit, forcibly liberated from confinement king Sudarsana of great energy.
I discuss Krishna's so-called "kidnapping" of Rukmini here, but my question is, what is the story of Krishna rescuing the king Sudarshana from "the Gandharas and ... the sons of Nagnajit"? The only Nagnajit I'm familiar with is the king who was the father of Krishna's queen Satya, but this book says that this is a different Nagnajit:
This Nagnajit was a king of Gandhara and had imprisoned Sudarshana and other kings. Krishna liberated them. There is yet another Nagnajit, who was king of Koshala. Krishna married his daughter Satya.
And indeed, this chapter of the Adi Parva of the Mahabharata mentions a king of Gandhara named Nagnajit:
Then was born the disciple of Prahlada, viz., Nagnajit, and also Suvala. And from Suvala was born a son, Sakuni, who from the curse of the gods became the slayer of creatures and the foe of virtue. And unto him was also born a daughter (Gandhari), the mother of Duryodhana.
So that makes Nagnajit Shakuni's uncle. In any case, why did Nagnajit kidnap a bunch of kings, and how did Krishna get involved in freeing them?
Are there any other scriptures that discuss this?
EDIT: Nagnajit of Gandhara is also mentioned in the Aitareya Brahmana of the Rig Veda, see my question here.