I'm having trouble with a pronoun, and some meanings. The passage seems to expect reader familiarity with the details of the Medea & Jason story.
Sed postquam Colchis arsit nova nupta venenis,
flagrantemque domum regis mare vidit utrumque,
sanguine natorum perfunditur impius ensis,
ultaque se male mater Iasonis effugit arma.
But after the new bride burned with Colchian poisons
and either sea witnessed the palace of the king burning
her unholy sword was ...