This page contains spoilers for the whole drama.
The hallows were created by Ma Gui and Fu You who were the Haixing and Yashou leaders of the allied forces who were at war with the extremist Dixingian Chieftain 10,000 years before the start of the show. They were made out of fragments of the meteor and meant to benefit all of Haixing; however, Ma Gui and Fu You said that even they can’t unleash the full power of the hallows. Da Qing had touched it accidentally and was affected by it somehow (one way is that he lives for 10,000 years) In the course of the war, the holy tools were stolen by the Chieftain's rebels. Shen Wei and Zhao Yunlan were able to retrieve them after they ambushed the rebels in a forest.
When the holy tools are together, they can open up a wormhole that connects all space-time. Zhao Yunlan is sucked into one such wormhole in the present-day SID lab and is sent back 10,000 years to the war. When the holy tools were brought together and activated in the past, another wormhole opened and he was sucked back into the wormhole where present-day Shen Wei had been waiting for him. Together, the two were able to look down into the clouds of the wormhole and watch events unfold before them, though Shen Wei tells Zhao Yunlan that it is dangerous to stay too long in the wormhole.
The Ancient Secrets Record
ZH: "There's a saying from when the heaven and earth began. People were displaced and the Black Cloaked Envoy was in power. He called for action..."
ZYL: "What is she reading? Why was the Black Cloaked Envoy's history mentioned?"
DQ: "'The Ancient Secrets Record'. Found it by your bedside drawer."
ZYL: "Didn't you say before that the stories in this book have all actually happened?"
DQ: "Strictly speaking, there are some discrepancies. It is about ten thousand years of history." (Da Qing tries to talk about his memories and is rebuffed.)
ZH: "Four pillars guard the four cardinals. The North-West's sky inclined. Kun Lun wrote a memorial. The Stone that deteriorates before it grows old, the Water that freezes before it becomes cold, the Wood that rots before birth, the Gold that melts before it is heated: These are all unreachable things. And thus they become the Four Hallows. If the sky does not collapse, and the ground does not sink, then the four hallows veil. Thereupon peace befalls."
ZYL: "You heard that, right?"
DQ: "Four Hallows. The four hallows is essential for the peace of both ends."
ZYL: "Not only so. The recording mentions Kun Lun who's on a par with the Black Cloaked Envoy and who has the power to seal all the four hallows. But the Black Cloaked Envoy has never mentioned this Kun Lun to me before."
This hallow takes one person's life force and gives it to another.
It is unclear how much it takes/costs the person, and what exactly it does to the other person.
Using it between Haixingren and Dixingren has unexpected side effects.
The Longevity Dial was in the possession of Li Qian’s family, and was passed down to her from her grandmother. After her grandmother had a stroke, she used it to save her life in exchange for her own but mentally her grandmother was never the same. Her grandmother’s residual energy hid in the dial after she died and came out to save Li Qian when she was being attacked by the telekinetic Dixingian, which Black Cloaked Envoy said is unheard of, though he does not know all of the dial's powers. The dial starts off as a small pendant when Li Qian has it but it is noted to be growing in size by Lin Jing and Zhao Yunlan when it is in the SID's possession.
This hallow can separate a spirit ("a person's essence") from their body and "solidify" it, so it lives on without the body. Directly after separation, the spirit might not be visible. [ep10]
It can also reunite spirits with bodies. (more information needed)
It can penetrate the earth and trap spirits in pillars. [ep11]
Sang Zan's and Gelan's tribe, the Hanga tribe, had this hallow in their possession. Sang Zan tried to revive Gelan with it after she had died, because he didn't understand its power. It turned her into a ghost instead. He buried her body with it in a field of flowers. [ep10]
SW: "Some appreciate me, some criticize me. Sharing the merit and questioning sins. Similar sounds echo each other, the same odors merge with each other. Exploiting a situation to one's benefit, by pushing the boat with the current. Choosing to soar or remaining in the deep, there is nothing to blame by advancing or retreating."
ZYL: Translate that for me, Professor.
SW: What it means is the Merit Brush's effects are that it determines the quality of a power. It can erase the traces of dark energy used by the Dixingren's and even increase their power. Just like Feng Qubing from before. He suddenly obtained dark energy to heal people. And when he used this dark energy, I could not feel it at all. Turns out, this is because the Merit Brush was at work. [ep23]
The fruit vendor name Wang Xiangyang had it as a family heirloom, rumored to bring luck when it is used to write.
Lighting the Lantern: [ep40]
ZYL: Then what would happen to this warrior?
ZXC: For now, no one has tried, but one thing for sure: after doing this, this sacrificer will die and come back to life for an infinite number of times, suffering the pain which is much more painful and lasting than burning.
ZYL: Who would be so stupid to do that?
What does it do?
Somehow ties all the other Hallows together, or at least keeps track of them - when the Guardian Lantern was extinguished 10 years ago, the Hallows were lost [ep36].