How did Mordred kill King Arthur?

No one is for Sure if he killed him or not But if you Read diffrent books and records he Fatally injuired But King Arthur later on killed mordred but is Severely wounded and is taken on a boat to avalon where Supposingly Sir gawain died later it is said that he was burried in avalon and his Wife guenievere was burried beside of him.

And others say he is there and waiting to return in great time of Great britan.

No one is for sure what Mordred used to kill arthur but if you look at some records is that he stabbed king arthur with a spear while king arthur Non stop stabbing him with Excalibar.