In early Vedic times Asuras were incorporated among the gods. Such important Asuras were Varuna and Mitra. In that period Asuras were thought to preside over moral and social phenomena while Devas were responsible for natural phenomenon. By the end of the Vedic period, Asuras attended more demonic role. Their negative character evolved over time. Certainly there were some Asuras who were quite evil, like Vritra but some of them were as pious as gods. Sometimes the Asuras are even more powerful than gods, winning all the wars against them and forcing them to flee off leaving their kingdom until they found some way to defeat them. In some cases, there are also instances when Asuras are known to work alongside the Gods to achieve a common goal like churning the ocean in quest of Amrita.
In Hindu Puranas and history, Asuras are described, as demonic persons while Devas are the Godly persons. According to Bhagavad Gita all creatures in the world partake either divine qualities ( Daivi Sampad) or the demonic qualities (asuri sampad). The Asuric qualities are pride, arrogance, conceit, anger, harshness and ignorance.
The famous Asuras are-Vairocana, Sumbha and Nishumbha, who were the king of Asuras. Mahishasura was another Asura who was killed by Goddess Durga.