"The Wanderer" is a poem originally written in Old English. It describes a man whose nation was defeated, and he lost his home, lord, fellow warriors, etc. and is now just a wanderer sailing over the cold sea, not knowing where to go. The poem can be found here.
The main character is the Wanderer, who was (from the poem) a warrior happy serving his lord. The Wanderer sails off to find his fortunes elsewhere, while thinking of his former life and bemoaning his fate.
Some themes in the plot are loneliness and sadness, which are portrayed in the Wanderer's story of how he got in his position. Another theme is faith, which is portrayed in the Wanderer hopes that God will have mercy on him.
Some literary devices are kenning and caesura. An example of kenning is "spirit-chest" for mind or "treasure-chamber" for thoughts. An example of caesura is "So I, often wretched and sorrowful".
The poem is an elegy because it is a lamentation for the Wanderer's dead fellows and for his lost home. Hence, the mood is sorrowful.