He formed a connection with the Amethyst Astral Soul. This is actually an Astral Soul condensed from the Emperor Amethyst Constellation, that symbolises the will of Emperors. It is the same as his brother, Chu Tianjiao.
As for the 2nd Astral Soul of Chu Chen, it took the form of a palm imprint. One could imagine that the constellation he condensed this Astral Soul from was most definitely a gigantic palm-type constellation.