Huaisha...the weapon with 'vicious nature'

We know that two of Chu Wanning 's holy weapons, Tianwen and Jiuge, came from Jincheng Lake as he told his disciples so. Then, where did Huaisha come to be his another holy weapon from? Is there anything mentioned in the story at all? From its name, Huaisha (怀沙)--Embracing Sand to Drown Oneself, could it be implied that it has come from under the sea? This has bothered me for quite some time. Anybody knows anything about this? Thanks in advance.