with swords flashing like lightning and crashing like thunder.

It should be noted in this regard different kennings have been used to describe a single object in different parts of Beowulf. For example ‘sea’ has been described as “sail road” and “swan road” in different sections of this Anglo-Saxon poem. Other good examples of kenning include: “the foamy-necked floater” (used for ‘ship’) and “sea wolf of the depths” (used for ‘Grendel’s mother’).

A kenning is a metaphorical phrase or compound used to describe a person, place, or thing. The words used to make up a kenning are generally very evocative and conjure up distinct images in the mind's eye.

There are numerous kennings in the epic Anglo-Saxon poem Beowulf. One such kenning comes in relation to the fearsome monster Grendel, who's been wreaking merry havoc upon the hapless Danes, regularly barging his way into Heorot and carrying off warriors whom he eats later on in his cavernous lair.

It is entirely appropriate, then, that Grendel should be described as "the guardian of crime." One can imagine Grendel standing at the entry to his cave hiding the evidence of the vicious crimes he's committed against so many Danish warriors. In guarding his cave, he's also guarding the evidence of his crimes.

Grendel is also described as "the shepherd of evil." A shepherd, of course, is someone who herds sheep. But they are also supposed to protect them from external threats, such as ravenous wolves. Grendel, then, is protecting evil in much the same way as he guards the evidence of his crimes. It's notable that the two kennings follow each other in the same sentence, an indication that they are closely linked.

A kenning is an aspect of Anglo-Saxon poetry where two words are combined to create a powerful, creative and evocative alternative word, which act as strong metaphors. The Anglo-Saxon poets played around with words in this way to experiment with the sound and feel of words and their works. There are plenty of examples in this famous Anglo-Saxon text, including "bone-house" for the human body, "battle-light" for sword and "wave-floater" for ship. Note the following kenning in the following quote:

In the end each clan on the outlying coasts
beyond the whale-road had to yield to him
and began to pay tribute. That was one good king.

The description of the sea as a "whale-road" is particularly evocative, as it helps create a powerful image of the sea as the habitat of enormous sea creatures, mysterious in their own right, such as whales, who use the sea as their "road" in the way that humans use roads. This helps highlight or augment the bravery of those who travel on these so-called "whale-roads," as they are choosing to travel with such enormous creatures.

Beowulf's battle with Grendel begins on line 750 when Grendel seizes Beowulf's arm, not knowing that it is Beowulf, nor knowing anything of his strength. However, he immediately realizes that Beowulf is not like the other men, nor any man he has ever encountered. The battle ends on line 823, when Grendel flees, and is concluded on line 836 when we are told that Beowulf has fully ripped off Grendel's arm.

Kennings are compound metaphors common in Old Norse and its derivative languages and literary cultures. One of the first encountered in "Beowulf" is "whale-road" in line 10, a metaphor for the ocean. At first glance these might seem frivolous and unnecessary to the casual reader—why not just say "sea"? Part of the answer lies in the fact that "Beowulf" originated as oral poetry, and thus it would serve the poet to have an array of imaginative phrases, with different sounds and length, to drive the telling more in the direction of art than a simple relaying of facts. Some translations attempt to preserve the extensive alliteration present in the original text, some of which is supported by kennings. For example,

syndolh sweotol·      seonowe onsprungon·
burston bánlocan·      Béowulfe wearð
gúðhréð gyfeþe·      scolde Grendel þonan
feorhséoc fléön      under fenhleoðu,

This section from lines 817-819 employs frequent kennings to construct a "s - b - g/th - f" alliterative scheme with one sound dominating each line.

Kennings used in the battle include:

• bone-adorned (probably meaning "armored")
• life-days and day-count (lifespan)
• sin-scather (using an archaic form of "scathe", meaning injury)
• bone-locks (muscles)
• slaughter-storm (massacre or gory battle)

A kenning, as you illustrated in the example in your question, is a poetic renaming or redescribing of the subject. Sometimes the kennings in Beowulf really jump out at the reader, sometimes they're harder to recognize, and sometimes it's just hard to tell whether or not something qualiifies as a kenning.

The second paragraph in Chapter 1 has at least four examples of kennings:

There was Shield Sheafson, scourge of many tribes,
a wrecker of mead-benches, rampaging among foes.
This terror of the hall-troops had come far.
A foundling to start with, he would flourish later on
as his powers waxed and his worth was proved.
In the end each clan on the outlying coasts
beyond the whale-road had to yield to him
and begin to pay tribute. That was one good king.

The exact phrasing will vary, of course, based on the translation that you're using. I'm using Seamus Heaney's translation. (The link to the opening to Book 1 is given below.) Kennings in this one paragraph include:

scourge of many tribes
a wrecker of mead-benches
This terror of the hall-troops
the whale-road (= the ocean)

I would encourage you to identify your own set of favorites (I really love that idea that "a wrecker of mead-benches," which might be rephrased today "a bar-room brawler," makes a good king!) If possible, pick and discuss examples from different parts of the poem.

The authorship of Beowulf is a total mystery, and even the country in which it originated is in question. Although it is widely studied as the oldest extant epic poem in the English language, it is quite possible that it was not composed by an Englishman. The fact that the main characters come from what is now Denmark and Sweden lend credence to the claim that the poem was written by someone from that area rather than from England.

At any rate, it is quite apparent that the poem was written by someone who lived in a society in which battle was a proving ground for manhood and the sea was a major part of everyday life. We see this in the figurative language of the period, the kenning. A kenning is a two-word metaphor that serves to describe an important noun in an interesting way. The following kennings from Beowulf are associated with battle and the sea:

Spear din: battle

Bone crusher: Grendel

Sea road: ocean

Whale road: ocean

In order to begin, one must understand the terms one is trying to identify. A kenning is simply another form of a metaphor. Essentially, a kenning (famous in Anglo-Saxon texts such as Beowulf) is a typically a two word phrase meant to elevate and beautify the language of the text. For example, battle sweat is a kenning for blood. Personification, on the other hand, is the giving of human characteristics to non-human/non-living things. For example, the sun smiled. People can smile; the sun cannot.

In regards to the text of Beowulf, many examples of kennings and personification can be found. Here are a few examples of each.

Kennings

God- All-wielding Ruler, Guardian of Heaven, The Wielder of Glory.(Chapter III, lines 66-68.)

Grendel- Horrible Hermit (Chapter III, 52), devil from hell (Chapter III, 62).

Beowulf- chief of the strangers, War-troopers’ leader (Chapter V, lines 1 and 2).

Mouth- word-treasure (Chapter V, line 2).

King- Giver of rings (Chapter 6, line 34).

Sea- wave-billows (Chapter 7, line 22).

Personification

"The lances stood up." (Chapter 6,line 10).

"If death overtake me." (Chapter 7, line 75)

"The mere fishes’ mood was mightily ruffled." (Chapter 9, line 51)

"My obedient blade." (Chapter 9, line 59)

As you may know, kennings in Old English/Anglo-Saxon literature (and other Germanic literatures) are usually compounds of two or more words, sometimes whole phrases, that stand for single nouns in Modern English.  For example, in several places in Beowulf where the poet refers to the sea, we see the kenning whale-road and swan-road or he might refer to a high-ranking queen as peace-giver, a poetic way of creating an image that would resonate with readers or listeners.  A listener, for example, might not be familiar with the word sea or ocean,  but he or she would understand what the whale-road means because everyone in that society was familiar with such images.

In the two lines you've asked about, the kenning is in line 545, "Heaven's Own candle," which stands for the sun--comparing the brightness of the light in the hall to the brightness of the sun.  In another section of Beowulf, the sun is referred to as "God's bright beacon," another kenning much like "Heaven's own candle."  

The epic poem Beowulf is the earliest surviving Old English work. It is believed to have been composed between 600-900 A.D., and then later transcribed by Anglo-Saxon monks in what is now known as England. We have no real idea who created the poem, which might very well be based on Norse stories that travelers or storytellers brought from the north.

Personification is the attribution of human qualities to a nonhuman entity. After Grendel's first attack on Herot, the poet (according to the Burton Raffel translation) says that:

Distance was safety, the only survivors
Were those who fled him. Hate had triumphed.

Hate is personified in this line. When the poet says that it "triumphs" he implies that hate is willful, in the sense that it can try to defeat something, like a human being would do.

Later, the poet tells us that as a result of his misery, Hrothgar is miserable. He uses personification when he says:

His misery leaped 
The seas, was told and sung in all
Men's ears.

We know that misery cannot actually "leap." The poet means that the story of Hrothgar's misery was told by travelers, eventually making it to Beowulf's ears. Since Beowulf lived across  the sea from Hrothgar, saying that misery "leaped" gives the reader (or listener, when the story was first being told) a nice image, a sense of action as the story is told, then re-told, until Beowulf hears it. 

In relation to Anglo-Saxon literature, a "kenning" can be defined as a metaphorical phrase or compound word used to name a person, place, or thing. Kennings abound in Beowulf; the poem's four-beat alliterative lines allow for a huge variety of them to be used. As well as "ring-giver," we have "battle-sweat" meaning blood. And in his brutal epic encounter with Grendel's Mother, Beowulf does indeed shed a fair amount of battle-sweat. Another one is "sail road" meaning the sea. If you think about it, a sail road is exactly what the sea is to Anglo-Saxon warriors like Beowulf. They routinely set sail upon the high seas to trade, fight battles, and explore new lands.

"Sleep of the sword" is a particular favorite of mine. You won't be surprised to discover that it means "death." Notice how two concepts have been yoked together: the sleep of death and the method used to bring it about. Like "battle-sweat" and "sail road," "sleep of the sword" provides us with a glimpse into Anglo-Saxon culture, where death, especially to brave and noble warriors, often came at the end of a sword.

Kennings are compound metaphors common in literature derived from Old Norse linguistic traditions. Functionally they appear may appear frivolous or unnecessarily abstract, but are intended to provide imagery and dexterity to the poet in order to avoid repeating words or to improve the narrative creativity of the work. "Ring-giver" is intended to emphasize the role of a king or leader as a benefactor and source of reward for good service, rings (or torcs, a type of collar or necklace) being a form of jewelry that often conferred status.

Kennings are found throughout "Beowulf" and comprise a significant portion of the text, both in narrative illustration and word count. "Ring-giver" is found on line 1102, and others in this portion of the text include "hate-bites" on 1122, referring to wounds, and "battle-light" on 1142, referring to a sword. 

While you don't mention which translation you are using, these phrases are all what are called kennings. A kenning is a two-word adjective prominent in Anglo-Saxon poetry. They give a distinct terseness to the poetic line and conjure a powerful image in the readers' mind. These do the same thing, all enhancing the sense of the intensity of battle.

The poem focuses on Beowulf's destiny as a warrior who serves as a saving force for Hrothgar's court and then, later, the Geats. He is destined to fight monsters like Grendel, Grendel's mother, and the dragon, who are "fate-cursed." Beowulf is fated to defeat Grendel, though he is himself fated to die in battle with the dragon. A destiny shapes these warriors' lives and cultures, and they must accept their fates with courage and loyalty to king or forever be labeled inglorious.

"Doughty" is a word that means fearless, and knowing the reality (or grimness) of death and its finality requires this type of courage and resolution. Rather than enter battle in a spirit of adventure and longing for fame, the poem's warriors grimly encounter their foes and their potential death, knowing that failure to do so is failure to uphold the foundations of the community, which is based on the comitatus, or a code of loyalty, bravery, and generosity.

In the end, the Geat warrior's unwillingness to face the dragon leaves them leaderless and subject to attack by other tribes, who will no longer fear the Geats.

Kennings are metaphors composed of vivid compound words or short phrases. Kennings are often found in Old English poems like Beowulf, and they add an element of intense description and beauty, highlighting the power of language to surprise and delight. Let's look at some examples of kennings.

“Light-flashing” refers to a bright object, something that reflects light. In the case of a battle poem, the kenning usually refers to a sword. We can easily picture the sunlight glimmering as a warrior raises his sword in battle, perhaps leading a charge or bringing the weapon down against an enemy.

“Grim-death” refers to death on the battlefield. The Old English word for “grim” may also be translated as “fierce,” so this is the death of a warrior in the midst of battle.

“Doughty-in-battle” is a brave warrior, a warrior who does not turn away from the enemy and who fights to the death for his lord. In ancient Germanic culture, courage and glory were more important than life or death. A warrior would risk himself to prove himself and win glory, even if he died in the process.

“Fate-cursed” refers to someone or something that is doomed. This being or object will not survive. One might say that in Beowulf, Grendel is “fate-cursed,” for as soon as Beowulf meets him in Heorot, he has no chance of victory or survival.

“Battle-grim” speaks about both the atmosphere of a battle and the attitude of the warrior. A wind, for instance, might blow “battle-grim” to suggest the coming of a fight. There is a fierceness in the very air. A warrior, too, may be battle-grim as he prepares for a fight from which he may not emerge alive. Yet he is ready; he is stern and fierce. He will meet whatever comes with courage.