Decribe the visitor who interrupts the feats?

That would be the Green Knight.

There bursts in at the hall door a terrible figure,
In his stature the very tallest on earth.
From the waist to the neck so thick-set and square,
And his loins and his limbs so massive and long,
In truth half a giant I believe he was,
But anyway of all men I judge him to be the largest,
And the most attractive of his size who could sit on a horse.
For while in back and chest his body was forbidding,
Both his belly and waist were becomingly trim,
And every part of his body equally elegant
in shape.
His hue astounded them,
Set in his looks so keen;
For boldly he rode in,
Completely emerald green.