Ashtaroth, Baal and Cankerworm

Ashtaroth, Baal, and Cankerworm are major antagonists in the Left Behind series. They are demonic entities who are agents of the Devil.

Background
Ashtaroth, Baal, and Cankerworm were originally three angels in heaven who served God. However, when Lucifer rebelled, they chose to serve him instead of resisting him, and fought alongside Lucifer in his rebellion against God. As a result, the three became the ⅓ of the angels that were cast from heaven, or demons; Lucifer became Satan. They continued to stay loyal to him throughout the history of the world.

Armageddon: Cosmic Battle of the Ages
The three demons, disguised as frogs, exit the mouths of Nicolae Carpathia and Leon Fortunato, with two of them exiting Nicolae's mouth (because he is indwelt by Satan) and one exiting from Leon's mouth. They immediately enter three lifeless Carpathia clone bodies after this, using them to gather the Global Community forces together to battle and overthrow Jesus Christ at Armageddon.

Glorious Appearing: The End of Days
However, Ashtaroth, Baal, and Cankerworm's plan doesn't turn out too well, as they are captured by the archangel Gabriel. Both Gabriel and Jesus judge the three Carpathia look-alikes for their crimes, before Jesus Himself sends them into the Lake of Fire for good.

Trivia

 * Ashtaroth, Baal, and Cankerowrm are the first beings to enter the Lake of Fire in the Left Behind series, with the second being Leon and the third being Nicolae, the latter two of which are the first humans to enter the Lake of Fire in the series.