Lulach was (the real) Macbeth’s stepson. He was killed in battle in 1058 – so presumably the story goes that this stone commemorates him.
The information on canmore makes it sound quite a strange shape, as it bulges out towards the top. It is 2.5 metres high and stands in the middle of a circular banked enclosure. A boulder inside the circle ‘has been carved at a relatively recent date probably with a cold chisel’ to make a stone ball.
Grinsell (in ‘Folklore of Prehistoric Sites..’) mentions a rumour that the stone (deliberately?) crushed a treasure seeker to death, so be warned.