Gladiator fights in massive arenas are a popular form of entertainment in Nilfgaard. Usually the fighters are slaves specially trained for this purpose, yet at times the arena masters pit humans against monsters. The shaelmaar Geralt had to fight in the tourney grounds was a present from the emperor to his relative, Duchess Anna Henrietta. It had been caught while young and prepared for its ultimate purpose by Nilfgaardian trainers. That does not mean it was tame, however – in fact, its wildness and inborn ferocity had not been dulled one whit. Like all creatures of its species, it was blind but could unerringly pinpoint its opponents' location using echolocation. The shaelmaar had thick armor and near impenetrable skin. It could also roll into a ball and charge its opponents. Its underbelly, however, was relatively vulnerable and became exposed when its charge failed.
In the end, though the fight was long and difficult, Geralt defeated the shaelmaar and, when the duchess called from the stands for him to finish the beast off...
If Geralt kills it:
...he duly dealt the final blow.
If Geralt spares it:
...he refused to kill the creature, not wanting to take a life merely to amuse a crowd.