"Do you really wish to know?" Spoilers from the books and/or games to follow!
The Battle of Sodden Hill, also known as the Second battle of Sodden took place during the first war with Nilfgaard. The hill itself is also known variously as "Mage Hill", "Kite Hill" or the "Hill of the Fourteen", because fourteen of the twenty two mages involved in the battle were said to have been killed fighting the Nilfgaardian army (at least according to official sources). A reported one hundred thousand participants fought in the battle, of which at least thirty thousand were either killed or wounded. The battle ended in a disgraceful defeat for Nilfgaard and marshal Coehoorn in particular.
Although there are fourteen graves, there are no more than thirteen bodies. Triss Merigold is said to be among the fourteen, but she survived if not unscathed. The names of the fallen (including Triss) are carved on a stone obelisk. Each year at Belleteyn, fires are lit to commemorate those killed on the hill.
Upon his visit to the site after the conflict Geralt was terrified that the 14th name on the hill was that of Yennefer. Fortunately for him the name belonged to another mage called Yoël Grethen whom he did not know.