Iorweth isn't one to talk about it himself a lot, therefore not much has been revealed about his past before he joined up with the Scoia'tael - non-human units of freedom fighters, outlawed by humans and their laws. He is somewhat about three centuries old – and thus rather youngfor an elf. (this is his age in The Witcher verse – it will be adjusted to fit other verses if necessary)
What is known is that he has long been fighting an unwinnable war, one that he was born into:Those Scoia´teal Units have not always been outlaws though. They were founded during a war humans against humans in the course of which the elves were recruited by one side in order to aid them - in exchange the Aen Seidhe were promised a piece of land where they should be allowed and able to re-found their own elven kingdom. Iorweth commanded one of the most renowned and fierce units of these Scioa´teal, they engaged in guerrilla warfare and did significant damage behind enemy lines.
In the course of this said war the Scoia´tael were betrayed and butchered by their human allies, those able to escape marked as bandits and made outcasts. This experience turned Iorweth into a fierce fighter against those who suppress and mistreat his people the Aen Seidhe and also others of the Elder Races (Gnomes, Drawves, Dryads...).