Corakinus was the Shadow King of the Shadow Kin and leader of the War of the Underneath. (TV: For Tonight We Might Die, Co-Owner of a Lonely Heart) He ruled from the Shadow Palace. (TV: Co-Owner of a Lonely Heart)
In 2016, he began sharing his heart with April MacLean, (TV: For Tonight We Might Die) who later defeated him in battle and took his place as Shadow King. (TV: Brave-ish Heart) After getting his title back, Corakinus and his entire race was killed by Charlie Smith, using the Cabinet of Souls, and April took over his body. (TV: The Lost)
After wiping out the population of Rhodia, Corakinus pursued the Rhodian prince Charlie and his Quill bodyguard Miss Quill to ensure that Charlie could not use the Cabinet of Souls to exact revenge on the Shadow Kin. He tracked them to the Coal Hill Academy on Earth, where he led the Shadow Kin in an attack on the fugitives. Ruthless in his hunt for the prince, Corakinus killed Rachel Chapman without provocation and severed Ram Singh's leg when he attacked him.
Miss Quill instructed April MacLean to fire a displacement gun, but Charlie knocked her out of the way so it wouldn't kill her. The gun hit Corakinus' heart, scattering it across the universe, but he kept on living, sharing April's heart.
Corakinus was ultimately confronted by the Twelfth Doctor, who had been summoned for help by Quill, having been the one who relocated her and Charlie. When the Doctor used the Shadow Kin's aversion to light against them, Corakinus and his force were forced back through the tear in space-time from where they came which the Doctor subsequently sealed. Corakinus warned, however, that he would later return. (TV: For Tonight We Might Die)
Upon returning to the Underneath, Corakinus was wracked with paranoia. He was supposed to be seen as a leader and a warrior, someone who would guide his people into battle but he carried the shame of being trapped into sharing the heart of a human schoolgirl.
Corakinus sought out medical help and assistance from other members of the Shadow Kin. Unfortunately for him however, none of the practices helped break his connection with April and he ended up killing all those who failed to find him a solution. Back on Earth, his constant experiments in meddling with their shared heart caused April to become more outlandish — alerting her new partner Ram to the dangers of the Shadow Kin's return.
Eventually, Corakinus was able to locate April — feeling that the only way to break their connection was to kill her. Corakinus opened a crack in time which led to April's back garden and April, along with Ram, entered with the desire of killing him before he killed her. (TV: Co-Owner of a Lonely Heart)
After reaching the Shadow Palace, April challenged Corakinus to a duel. They fought using their scimitars and in the end April beat Corakinus but decided to spare his life as they still shared a heart. April announced that she didn't need to kill Corakinus as the rest of the Shadow Kin had already seen her defeat him and therefore she would be proclaimed the new King.
Using her new army, April had Corakinus imprisoned. While imprisoned Corakinus removed the anchor between his and April's hearts, meaning that while they were still connected the connection was weaker. (TV: Brave-ish Heart)
Due to April neglecting her role as King of the Shadow Kin it wasn't hard for Corakinus to reclaim the title. In control of his army again he decided that it was time to destroy his enemies by letting the Shadow Kin kill everyone on Earth. However, to do this he needed to make sure that April was safe - as she still shared a heart with him.
In order to force April into giving herself up he decided to begin executing the gang's closest family members. He executed Ram's father Varun and Tanya's mother Vivian. He even tried to kill April's mother Jackie MacLean but due to her having part of his soul inside her he couldn't.
After a stand off, Corakinus put a shadow onto Charlie's heart. He later went on to attack Jarvis and Damon Adeola, but was prevented from killing them by a heavily pregnant Quill. After this he captured Matteusz Andrzejewski and used him as a bargaining chip to get April to come with him. She agreed and was in his grasp, but she found out his true plans to kill the human race. Charlie shot April and, as they both shared the same heart, Corakinus died too. This led to Charlie becoming the Shadow King.