Akampana was one of the demons and a trusted lieutenant of Ravana. He was killed by Hanuman in the Ramayana. It is mentioned in the Sarga 56, Yuddha Kanda of Ramayan.
When Rakshasa Dhumraksha was killed by Hanuman, Ravana asked Akampana to enter the battlefield.
He led a vast army of demons and fought fiercely against the Vanarasena (monkey army). But majority of the demons were killed by the Vanarasena headed by Hanuman.
Akampana was furious and started attacking monkeys like a mad man. Hanuman countered him barehanded.
Hanuman threw a huge boulder on Akampana. The demon reduced the huge stone into powder with his arrows.
Hanuman then uprooted a tree and whirled it like a wheel killing many demons.
Akampana sent fourteen arrows at Hanuman but they were all blocked.
Hanuman then uprooted another huge tree and hit Akampana on his head.
This resulted in the death of Akampana.