The Loves of Hercules

The Loves of Hercules is a 1960 ItalianFrench international coproduction epic fantasyfeature film starring Jayne Mansfield and her then husband Mickey Hargitay. The film was distributed internationally as Hercules vs the Hydra.

Hercules admires Deianira and her bravery. When escorting Deianira back to her capital they come across a band of peasants who have been attacked by a monster. As Hercules seeks the monster their cattle is stampeded and Hercules kills a wild bull with his dagger. Arriving in the city Hercules discovers Deinaira is betrothed to Acheloo and leaves.The resourceful Licos hatches another scheme where he murders Acheloo with the dagger Hercules left behind in the bull not only casting guilt on Hercules but making Deianira available for marriage to him. The actual murderer is sent by Licos to the Underworld with the plan that Hercules can only prove his innocence by going to the Underworld where no mortal has ever returned from due to the monstrous Hydra. ........

Source: Wikipedia