Achelous told Theseus and his men the story of his battle with Hercules. Achelous and Hercules were fighting over Deianira. Achelous had been in human form, but he couldn't outmatch Hercules, so he changed to a snake. From the snake, he transformed himself to a bull, and Hercules pinned him to the ground and broke off one of his horns. The broken horn was made the cornucopia by his water nymphs. .
Theseus and his men left Achelous' cave the next morning and Achelous turned back into water. .
Book 9: Hercules, Nessus, and Deianira .
When Hercules was taking his wife to his home in Tiryns, they came to a swollen river. Nessus, a centaur, offered to help Hercules get Deianira across the river. Hercules swam across and heard Deianira yelling for him. Assuming that the centaur was kidnapping her to rape her, Hercules shot him with an arrow tipped with Hydra's poison. The centaur, seeking vengeance, gave his shirt covered in Hydra's poison to Deianira and told her that it was a talisman that would renew love. .
Years later, when Hercules had completed his labors and was still far from home, Deianira heard from "Rumour who talks and loves to tangle true / With false, and from near nothing flourishes / On her own lies" Book 9 -- Hercules, Nessus, and Deianira, line 141-3 that his heart had been captured by another woman. She sent him the shirt not realizing that it was poisoned. Hercules, unaware of its taint, wore the shirt.
Book 9: The Death and Apotheosis of Hercules .
The shirt destroyed Hercules' mortal body and he suffered awfully from Hydra's poison. He ran into the woods and built a funeral pyre for himself and climbed atop the flames. Jove allowed the mortal part of his son to burn away and then made him a god and placed him among the stars. Alcmena talked to Iole, Hercules' pregnant mistress, about her son's birth.
Book 9: The Birth of Hercules .
Alcmena had a difficult time delivering Hercules because Juno wanted to punish her rival for Jove's attention.