Yennefer cried blood when she let her chaos explode, just like any other mage would have, because she had gone beyond what her powers allowed.
Yennefer cried blood during the Battle of Sodden Hill in the first season, because she had completely over-extended herself and her powers to try and protect the continent (in the books she even went completely blind afterwards).
The Witcher: Why did Yennefer cry blood?
Netflix’s live-action The Witcher series certainly came out of the gates swinging. The battle at the end of the first season of the show, which has now become known as the Battle of Sodden Hill, still stands out as one of the most brutal events in the series to this day.
During this battle, Yennefer, alongside about 60 other mages (and some slow-approaching backup from the northern armies) took to defending the Elven keep from Nilfgaard.
However, the mages were highly outnumbered and overpowered in this entire fight and after seeing many of her own defeated in the battle, Yennefer decided that it was up to her to save the continent.
Stopping a powerful force like Nilfgaard took all of the power that Yennefer could muster, and she eventually had to let her chaos explode. This over-exertion of her powers is ultimately what caused Yennefer to cry blood.
Crying blood was not the end of Yennefer’s injuries
Although Yennefer giving in to chaos helped her side win at the Battle of Sodden Hill, this incredible showcase of her powers did not come without a cost. After the battle, Yennefer was taken prisoner by Fringilla and later a group of Elves.
It is also eventually revealed that she lost her powers in this process and Yennefer ends up spending much of the second season trying to get her powers back.
The Battle of Sodden Hill was even more grueling in the books
It may have been difficult for The Witcher fans to watch The Battle of Sodden Hill play out on their screens at the end of the first season, but this battle was even more brutal in the books.
In the original story, Yennefer’s injuries even go one step further than crying blood, as she is actually blinded during this fight by Fringilla.
Other times that mages have cried blood
Since Yennefer’s great explosion in the first season, these ‘blood tears’ have actually featured a few more times in the series when mages were forced to go beyond the scope of their powers.
This includes an instance in which Ciri over-exerts herself trying to repair a bridge in the second season. As well as an instance in the most recent, third season where Tissaia seemingly gets overwhelmed seeing the extent of Ciri’s powers.