Which demon king abducted Sita in the Ramayana?

The demon king who abducted Sita in the Ramayana is Ravana.

Ravana is the rakshasa king of Lanka and the principal antagonist of the epic. He is portrayed as a powerful ruler, warrior and learned devotee of Shiva. His abduction of Sita begins the chain of events that leads Rama to gather allies and wage war against Lanka.

Ravana’s plan depends on deception. His uncle Maricha disguises himself as a magical golden deer, distracting Rama from the forest dwelling. When Lakshmana also leaves, Ravana approaches Sita disguised as an ascetic and carries her away. The vulture-king Jatayu tries to stop him but is mortally wounded.

Sita is held in the Ashoka Vatika in Lanka, where she rejects Ravana’s advances. Hanuman later finds her and carries news of her location back to Rama. Rama’s forces cross to Lanka, defeat Ravana’s army and kill Ravana in the climactic battle.

Mahishasura is another demon defeated by the goddess Durga. Indrajit is Ravana’s son, while Kumbhakarna is his giant brother; neither is the abductor in this question.