Tone in Beowulf
The loss of so many of his guard humiliates and saddens King Hrothgar, who gets no respite from the first attack because Grendel remorselessly strikes again the very next night. Since Hrothgar and his people are unable to have time to properly grieve, Grendel's relentless attacks help to provide an atmosphere of despair and helplessness around Hrothgar's Hall.
An excellent example of personification, the poet uses it to give the fire human qualities which add to this emotional scene in the tale. Additionally, personifying the fire as a demon is much closer to the pagan than the Christian belief system.
The description of the moor, or bog, details the surroundings as exactly the opposite of the homes of humans--cold, dark, and mysterious. The lake in particular adds to this unknown and treacherous environment, particularly its indiscernible depth of the lake and that a strong deer would rather be killed than enter the mere. The poet's description sets the mood for Beowulf's encounter with Grendel's mother by establishing that he will have to enter her realm.