Fei took out a treasure that looked like a circular badge but it is a seal or rather a mark. The only mark that can allow anyone to understand Dao of Reincarnation.
Of course, he can directly use it for understanding Dao of Reincarnation but the effect given by the Eyes of Reincarnation will be a lot than the effect by directly understanding the Dao of Reincarnation.
If he uses Mark of Reincarnation to understand Dao of Reincarnation then he will only gain fifty percent of true knowledge about Dao of Reincarnation, not to mention he needs to comprehend that knowledge.
But Eyes of Reincarnation gives him complete knowledge about Dao of Reincarnation. Not only that but whenever he tried to comprehend the Dao of Reincarnation, he can have talent from those eyes to understand it.