Iñaki de Juana Chaos has long provoked volcanic feelings in Spain. The 51-year-old former head of Eta’s “Madrid cell” was one of the most violent and committed members of the armed Basque separatist group during the 1980s.

His attacks included machinegunning a car carrying three soldiers, killing a rear-admiral and planting a car bomb that ended the lives of 12 policemen. A US citizen also lost his life when he was caught in a bomb blast by de Juana’s cell.

Another member of the cell, who later repented, once described how de Juana would personally select the victims and pick which members would shoot them. Even then, he said, de Juana would sometimes appear on the scene in a white cap and sunglasses to rake the vehicle with machine-gun fire.

After he was caught in 1987, de Juana was sentenced to 3,000 years in jail for his role in 25 killings. But under sentencing guidelines then in force, he had to serve only 18 years and was due to be freed about two years ago. Other Eta prisoners have been released by previous governments, but none has quite managed to acquire the notoriety of de Juana.

In 1990 police foiled an escape plot by him worthy of a James Bond film. Collaborators were caught in the final stages of a plan to fly a helicopter painted with Red Cross colours into the high-security prison where he was being held.

His fellow conspirators had taken helicopter flying lessons and secured a high-speed dinghy for his escape.

Throughout the decade stories appeared in the Spanish press that further fuelled his notoriety. In 1997 authorities intercepted a letter in which he expressed his joy at seeing the televised funeral of two Seville politicians — a married couple — murdered by Eta. The following year he reportedly asked the director of the prison for champagne and prawns to celebrate the murder of another politician.

De Juana is also one of the few identifiable faces of Eta, listed as a terrorist group by the US and the EU. On the few occasions that they have appeared in videos, Eta leaders have worn sinister-looking masks and berets.

To Eta’s victims, who are organised into a powerful political force, de Juana is an especially hateful character. As one of the group’s former hardliners, he is also a potent symbol for jailed Eta members, who still wield considerable influence over the group and inspire new recruits.

Once he is free to walk the street in something over a year’s time, where will he go?

He has said that he wants to dedicate himself to writing, but he is unlikely to fade into obscurity in the Basque country. Far-right figures have threatened to kill him and many Basque towns will not want a notorious killer in their midst.

Times Recommends

• UN may halt 400-year-old 'freedom of the seas'
• Where the Rosetta belongs can’t be set in stone
• Gas deal on hold, but Ukraine may pay in the end