I was reading about the Scottish independence during the period of Wallace and Robert the Bruce between 1200-1300.I remember I did read something about a warrior, who would sneak into a fort at night and behead everyone, he was called the english nightmare.Later, however, I tried to search more about that person but I couldn't find absolutely nothing.
That sure sounds like Sir James Douglas, aka 'The Blak Dowglas' or 'The Good Sir James'. His father, Sir William Douglas, had been an "early adopter" supporter of William Wallace; during Wallace's campaign, the elder Douglas was captured by the English and died in the Tower of London. The Douglas lands and castle were awarded to Robert de Clifford, newly appointed guardian of Scotland under Edward II of England.
Sir James responded by throwing in his lot with claimant to the Scottish throne Robert the Bruce, who had just been crowned King of Scots by his followers in 1306. The Scottish forces were few in number, and relied on guerrilla tactics to fight the more numerous occupying English troops. The first incident that turned Sir James into a boogeyman in the eyes of the English was the so-called "Douglas Larder", where Sir James led a party of his kinsman to retake Castle Douglas on Palm Sunday of 1308. His forces attacked while the English troops were at Sunday service, surrounding the church and killing the English soldiers. The bodies were taken to the now-empty Castle Douglas and burned-- wine casks and wine from the cellars provided wood and accelerant. To prevent the English from retaking the castle, Sir James and his forces poisoned the wells with salt.
The nocturnal stealth attack you mention sounds like the capture of Roxburgh Castle by Sir James and the Douglases in the winter of 1314. Roxburgh Castle was well garrisoned and provisioned for an anticipated siege by Bruce's supporters. With the help of sympathetic locals, the Scottish forces surrounded the castle using cloaks and branches as cover. Some contemporary accounts indicate that the guards at the castle mistook them for livestock rustling about in the undergrowth. That night, the Scots used modified hook ladders made out of rope to scale the walls and ambush the garrison and destroy the castle.
Sir James' men went on to wage a campaign of terror along the English border. Between his earlier exploits and his border raids, he became a feared figure in England. A nursery rhyme of the period runs:
Hush ye, hush ye, little pet ye,
Hush ye, hush ye, do not fret ye,
The Black Douglas shall not get ye.
Sources:
Robert The Bruce, by Ronald MacNair Scott
James the Good: The Black Douglas, by David R. Ross
A History of the House of Douglas, by Sir Herbert Eustace Maxwell