So, here's why. As mentioned before, the Lombards and their allies, the Avars, joined forces and invaded the land of the Gepids in 567, which resulted in Gepids' complete defeat. After that, their king was decapitated and Rosamund, the king's daughter, along with many other Gepids, was taken as a prisoner of the Lombards. Later, Alboin forced Rosamund to marry him following the death of his first wife.
In his writings, Paul the Deacon, the historian of the Lombards, reported that Alboin routinely abused his new wife and mocked her. In June of 572, he also stated that at a royal banquet in Verona, Alboin ordered Rosamund to drink from his cup, which was made from the skull of her dead father.
This incident apparently ignited Rosamund's determination to take revenge on the Lombards king. To obtain this goal, she persuaded Helmichis, her lover and also the king's arms bearer, to take part in a plot to eliminate Alboin.
Helmichis then suggested using the aid of a bodyguard named Peredeo to accomplish the assassination. Peredeo, however, refused to help. Rosamund then tricked Peredeo by disguising herself as a servant at that night and had intercouse with him. After realizing he had committed adultery with the king's wife, Peredeo eventually agreed to take part in the assassination attempt in fear of the king's punishment.
"Rosamund forced to drink from the skull of her father", a 17th century painting by Pietro della Vecchia