A weight spell causes the object on which it is cast on to hold "the weight of a thousand ages", making it unbearable for anyone to hold. This spell can be telepathically activated by the witch or warlock who performed the incantation.
Morgana used it on Arthur's sword, so that the King would have been defeated by Derian. It had been stolen by Agravaine who then gave it to Morgana. She had prepared three piles of burning pieces of wood, placed to make the shape of a triangle. She knelt in the middle of it, and begun to chant these words: "Efencume ætgædre, eala gastas cræft ige. Ecgahefigie; ahefigie mid þære swærnesse þusendageara". As her eyes glowed gold, she held up the sword and its blade caught fire. Then she said: "Ofercume þone sweordboran". After a few seconds, the sword ceased to burn and Morgana gave it back to Agravaine, who secretly returned it to the King.
The next day, during Arthur and Derian's duel, Morgana focused her powers on the sword and activated the spell. Arthur was no longer able to wield it, and would have been killed if Merlin hadn't used his magic to stop Derian and save his friend (His Father's Son).