Question

Which element of medieval literature does the disappearance of the sword into the lake represent

Answer 1

The Supernatural Element

The portion of the story you are referring to is the part where King Arthur throws his sword, Excalibur, into a lake where the Lady of the Lake inhabits.  After he throws Excalibur into the middle of the lake and before it lands into the water the Lady of the Lake catches it and takes it into the lake with her.   The Lady of the Lake can be considered a supernatural being, thus contributing to the supernatural element, an element which is common in medieval literature.