So what really happened to Lan Tian? Let's backtrack.
Lan Tian first gets bitten by the poisonous moths. Now here's where the questioning begins? What does this poison do? Does it cause death? Memory Loss? Headaches? Unfortunately, we don't know. Han Fu Zi only said that these were very poisonous moths. The chance of death is very high BUT Lan Tian didn't die BUT it could've also been that Meng Tian Fang's memory and his water crystal saved Lan Tian.
Next, only Lan Tian heard "Meng Tian Fang" breathing. Li Li could not hear it at all. This suggests there's a connection between Lan Tian and "Meng Tian Fang". Lan Tian is Meng Tian Fang's reincarnation. BUT in order for that statement to hold true, Meng Tian Fang must have died 2000 years ago in the fire with Dong Er.
Lan Tian retrieves the water crystal from Meng Tian Fang and then a ghastly white floating thingy flies into Lan Tian. Lan Tian then becomes "Meng Tian Fang". What is that white floating thingy? Is it Meng Tian Fang's soul? Or is it just his memories? Following the logic from the above paragraph, if Meng Tian Fang died 2000 years ago, then that thingy is not his soul. It's most likely his memories. In fact, there's a person at ATW's Baidu Forum that suggests that Meng Tian Fang's memories were absorbed by the water crystal and when Lan Tian touched the water crystal the memories were released and entered Lan Tian's body. The reason it entered Lan Tian and not Li Li is because the memories belong to Lan Tian's previous life. So that means, there was never a revived Meng Tian Fang and Lan Tian has always been Lan Tian.
English Translation of yiluyoun24510's interpretation of Lan Tian's Identity
Lots of friends who've watched "A Terracotta Warrior" believe Lan Tian was possessed by Meng Tian Fang's soul, at first that was what I thought too, but afterwards a lot of circumstances changed my mind.
1. After Zhu Li Li died, Lan Tian cut his wrist to die in the name of love, if at that time he was really possessed by Meng Tian Fang, then due to the powers of the longevity pill, even if he died, his soul would continue to live on, even possessing the body of his third life. But in the third life, Meng Tian Fang's soul never appeared, and thus I believe the longevity pill's powers did not work, so this thing about Meng Tian Fang living 2000 years and possessing Lan Tian does not exist.
2. In the ending, third life Luo Kai Ping saw with his own eyes the skeletons of Lan Tian and Zhu Li Li, which proves that longevity people do not exist in this world, the person who originally entered the Emperor's Mausoleum was Lan Tian and not the revived Meng Tian Fang. I think the water crystal worked its power, remember that Lan Tian was bitten by the poisonous moth, afterwards, he suddenly forgot why he was in the mausoleum. I think this poisonous moth makes people lose their memory, and the water crystal could preserve memories from previous life. Because when Lan Tian took Meng Tian Fang's water crystal and Zhu Li Li's crystal, two become one, and Meng Tian Fang's statue collapsed, from there, Meng Tian Fang's memories were released, and at the time Lan Tian's memories because of the poison, were lost, Meng Tian Fang's memories entered Lan Tian's body, making him lose his willpower and obtain Meng Tian Fang's willpower. So I think Lan Tian is Lan Tian, he is Meng Tian Fang's reincarnation, he received Tian Fang's memories, but is not Tian Fang. The main guy character of the second life was Lan Tian all along.
3. This would also explain Zhu Li Li's mystery. Although, the water crystal could protect one's memories, but when the first life Han Dong Er ran into the fire, her crystal fell, thus her memories were not preserved, that's why the second life Zhu Li Li, from start to end, never realized she was Han Dong Er. And back then Tian Fang was turned into a terracotta warrior with the water crystal so his memories were preserved. Hence, Han Dong Er and Meng Tian Fang already died 2000 years ago, this matter of long living souls do not exist. The second lifetime is about Lan Tian and Zhu Li Li's love story, and not Meng Tian Fang and Zhu Li Li.
This is just my personal thoughts, you guys all have your own opinions, this writing is for reference.