One of the devils sent by Satan to attack the Christian army makes the most clever move, as it will turn out: he 'inspires' a wizard called Hydraotes, who also happens to be the king of --- nice question: which city? In Gerusalemme Liberata it was about no less than Damascus. In the manuscript of Gerusalemme Conquistata, Tasso wrote Samosata (again in current Syria), then Sidon (in Lebanon), and finally Maraclea (on the coast; it would become a place ruled by the Knights Templar, then destroyed by the Mamluks in 1271).
Hydraotes has a niece called Armida. The original text says "nepote," "nipote" in nowadays Italian, that is a sadly equivocal term as it can mean both niece and granddaughter. In this case, circumstances lead to adopt the former solution.
The devil puts this project in the head of the wizard: Send the super-sexy Armida as a 'diversionary' among the Crusaders. And now one of the most fascinating episodes in Italian literature begins.