In the end, Billy had to use half of his mana to create that massive death trap. He thought that he would use all of it, but since his Earth magic was so powerful now, that was the result and he liked it, more or less.
When morning came, Billy was pretty tired. Not physically, but mentally since that kind of work was different than usual. Fortunately, it didn't take long for the fruits of his labor to appear.
You obtained 200 experience points.
The skill Earth Manipulation obtained 100 experience points.
You obtained 200 experience points.
The skill Earth Manipulation obtained 100 experience points
…