Until the cloud-murk he moved towards it
Until it shone above him ,a sheer keep
of fortified gold. Nor was that the first time 
Hhe had scouted the grounds of Hrothgar's dwelling.

The last line is the longest of the section;it ,supported by the punctation and two enjambements (line 5 ) , helps to a more speed of narration that suggest the Grendel's speed in scouting the grounds. The word "until" is repeated in two lines created an anaphor ,again increasing speed.
The narrator describes the high hall that appears of fortified gold.High hall has a great value and it represents the light  in contradiction with the kenning "cloud-murk" that remands to Grendel's movements and  fear that he creates.
In addition,information are at the end of the lines, that explains why inversion of word order focuses the attention on the words" the fist time", supported by past perfect that means an action happened before another action in the past .
The alliteration of sound "sh" is to focuse the attention on the word "sheer"  . It call the next line connected with a enjambements.