The Passing of Arthur

In this last idyll, Bedivere accompanies Arthur to his battle with Modred as his only remaining knight.  In the battle, Arthur charges at Modred who strikes him on the helmet.  Arthur slays Modred with Excalibur, though severely wounded himself.  Arthur, knowing he is dying, tells Bedivere to throw Excalibur into the lake.  Twice Bedivere disobeys and hides the sword, thinking it should be kept as a relic.  However, after Arthur threatens to slay Bedivere, the latter throws the sword into the lake and sees a hand reach up and grab it.  Arthur, realizing the end is near, has Bedivere carry him to the water's edge, where they are met by a barge with three queens.  Bedivere places Arthur in the barge, which sails for Avilion.

The final battle between Arthur and Modred takes place at Lyonnesse, which is the homeland of Tristram.  Tennyson provides an unattractive description of the location:

Then rose the King and moved his host by night,

And ever pushed Sir Modred, league by league,
Back to the sunset bound of Lyonnesse--
A land of old upheaven from the abyss
By fire, to sink into the abyss again;
Where fragments of forgotten peoples dwelt,
And the long mountains ended in a coast
Of ever-shifting sand, and far away
The phantom circle of a moaning sea.

It is in this land of abyss that Arthur must fight his last battle.  He goes outside the Camelot bubble to defeat Modred.  He defeats evil on evil's home turf.  In a land full of evil, purity is victorious.  There remains no other action other than for him to be carried to Avilion.
 
The above painting is Bedwyr and Dying Arthur (1862) by John Duncan.