这是there存在句结构，不过不及物动词rise 替换了be.

There形式主语 rose谓语 in his imagination地点状语 visions of a world empire真正主语.

vision在此为可数名词，意为[countable] an idea or a picture in your imagination

He had a vision of a world in which there would be no wars.

I had visions of us getting hopelessly lost.

a vision 一个画面 visions 许多画面。

脑子里呈现出很多画面就可以用visons. 没有必要改为单数。