First, there had to be a medium—an item intimately connected to Emperor Yu, and the connection could not be too distant. For instance, if Emperor Yu had casually forged some weapon, trying to use it as a medium to summon Emperor Yu's Undying Dao Shadow would be nothing short of a fantasy.
Forget Meng Zhao, who was only a Celestial being martial artist. Even someone in the Divine Demon Realm could never accomplish it; too many factors were involved.
Only Great Xia national treasures, such as the Dragon Sparrow Saber or the Emperor Yu Battle Armor—divine weapons personally and painstakingly forged by Emperor Yu—could serve as mediums.
For example, the Azure Dragon Battle Armor Meng Zhao wore was one of Emperor Yu's Five Armors. With Meng Zhao exerting himself fully, he could summon one-fifth of Emperor Yu's Undying Dao Image's power. That alone would place him at a level invincible across the four seas and eight wildernesses.
As for the price, it came in two parts.
First was the medium itself. During the repeated process of linking to and summoning Emperor Yu's Undying Dao Shadow, its spirituality would be worn away until it ultimately returned to extinction.
In other words, this medium could not be used to communicate with and summon him without limit. There was a cap on the number of times it could be used, and the damage to the divine weapon itself was immense.
Moreover, as for the martial artist himself, repeatedly summoning Emperor Yu's Undying Dao Shadow would cause the Primordial Spirit to be tainted by the Undying Dao Shadow's power. In the end, one's power would become impure, leaving one stranded at one's current martial realm and unable to break through to a higher level.
It was like poisoned honey. Its sweetness was beyond question,