9. Save that from yonder ivy-mantled tow'r
10. The moping owl does to the moon complain
11. Of such, as wand'ring near her secret bow'r,
12. Molest her ancient solitary reign.
He is saying it is all very quiet and peaceful (stanza 1 and 2) except for the sound of the owl, who is complaining to the moon about those who disrupt the solitude of her domain. Gray's note about this stanza is: "Molest her ancient solitary reign. - bother the owl while it keeps watch over the churchyard and countryside." (While you're at it, note the use of personification here). The solitary hooting of the owl probably adds to the mournful feeling.