whilom > in ancient times
4 Before angry gods, and cruel sky, 5 Upon you heaped a direful destiny,
direful > dreadful, terrible
6 What boots it boast your glorious descent,
boots it > does it avail to boast > boast of
7 And fetch from heaven your great genealogy,
heaven > (Olympus, since the founder of Troy, Dardanus, was a son of Jupiter. Furthermore, Aeneas, who ruled Troy after the fall of the house of Priam, was a son of Venus)
8 Sith all your worthy praises, being blended,
Sith > Since blended > stirred up; hence: rendered turbid, obscured, spoiled
9 Their offspring has embased, and later glory shent.