Kenning is a figurative expression that uses two or more words to refer to a single object or concept. In this case, "sword-feast" is a kenning for battle or war.
The image of a "feast" is used to evoke the violence and bloodshed of battle, where the "guests" are slain and the "food" is their blood.
Here are some other examples of kennings:
* Whale-road for sea
* Battle-sweat for blood
* Sky-candle for sun