I need to understand the following first two lines of the "Ode". It is about death of King Arthur:
"Yet in vain a paynim foe
Armed with fate the mighty blow:
For when he fell, the Elfin queen,
All in secret and unseen,
O'er the fainting hero threw
Her mantle of ambrosial blue."

Can someone explain it in prose? Do you think it means that a pagan foe tries to kill Arthur who is armed with fate and who is a mighty man?

The other sentence is:
"... the son of Uther, named Pendragon, a title given to an elective sovereign, mount over the many kings of Britain."

Couldn't understand what he means by "mount over" here.

Thanks in advance for your help.