But Cepheus left before the guilty room,
With grief appealing to the Gods above,
Who laws of hospitality approve,
Who faith protect, and succour injur'd right,
That he was guiltless of this barb'rous fight.
"Thee" refers to Iapetides the bard (a lyric poet) whom Pettalus scoffs at, loathes ("find repugnant"), and scorns ("look down on with disdain"), because the bard is defenseless and sings of peace, while Pettalus is armed and wants to kill.
Thee, when fierce Pettalus far off espied,
Defenseless with thy harp, he
Go; to the ghosts thy soothing lessons play;
We loathe thy lyre, and scorn thy peaceful lay:
And, as again he fiercely bid him go,
He pierc'd his temples with a mortal blow.
"Portend" and "bode" are synonyms and both usually indicate something ill rather than good. In the example sentence, the dark clouds indicate more storms are coming, but the tyrant is actually using the weather as an excuse to lure the Muses into his house in order to rape them. But on seeing a sign of his intentions, the Muses transform themselves into birds and escape.
Stop, stop, ye Muses, 'tis your friend who calls,
The tyrant said; behold the rain that falls
On ev'ry side, and that ill-boding sky,
Whose low'ring face
portends more storms are nigh.
Her varied members to a fluid melt,
pliant softness in her bones is felt;
Her wavy locks first drop away in dew,
And liquid next her slender fingers grew.
The body's change soon seizes its extreme,
Her legs dissolve, and feet flow off in stream.
In Latin, quondam="who once"--the quondam child once would not have minded his mother's touch, but transformed into a very small type of vermin (specifically, an eft or newt), he is scared and shuns ("avoid and stay away from deliberately") her by running into a crevice.
Amaz'd the dame the wondrous sight beheld,
And weeps, and fain wou'd touch her
Yet her approach th' affrighted vermin shuns,
And fast into the greatest crevice runs.