In the year 1069 Edgar Aetheling fled north to the court of Malcolm III seeking refuge from the Norman invaders of England. With him he brought his two sisters and the elder of his sisters, Margaret brought with her the holy relic that was to become known as the Black Rood of Scotland.
A rood is of course simply a crucifix symbolizing the cross on which Jesus was crucified; and the Black Rood was no different, being an ebony crucifix (hence the black) in which was set a piece of the true cross which was kept in a casket in the shape of a cross ornamented with gold and set with large diamonds. Where exactly it came from is uncertain, Margaret possibly brought it from Hungary where she was born or it possibly came from Waltham Abbey (per the Catholic Encyclopedia).
Margaret was to later marry Malcolm III and as queen of Scotland she became responsible for a major re-organisation of the Scottish Church and the endowment of a number of religious foundations. She died of grief in 1093 after hearing of the death of her husband and son at the battle of Alnwick and is said to have died with the Black Rood clasped in her hands as she recited the fifty-first psalm.1
The Black Rood then fell into the possession of her sons who fled Edinburgh Castle with her body as Donald Ban seized control of the kingdom. It was then presumably taken with them into exile in England, but returned to Scotland when her sons eventually managed to reclaim their inheritance, probably during the reign of Edgar but most certainly by the time her son David's turn came in 1124 as it was David who in 1128 founded Holyrood Abbey in honour of the holy relic of the Black Rood.2
The Black Rood was later transferred to Edinburgh Castle probably because it was thought to have been a more secure location given the threat of civil war that emerged in Scotland following the death of Alexander III in 1286. There it was seized by Edward I in 1296 along with the Stone of Destiny and whatever else he could get his hands on.
Edward carted the Black Rood around with him and used to add a certain amount of gravitas to the process of extracting the various oaths of fealty recorded in the Ragman Roll which he required from the various members of the Scottish nobility and clergy, before sending it for safekeeping at Westminster. There it stayed until 1328 when in accordance with the Treaty of Northampton it was returned to Scotland and was regarded very much as an heirloom of the kingdom.
When David II decided to invade England in 1346, he took the Black Rood with him, no doubt believing that possession of such a holy relic would guarantee him victory. David was unfortunately defeated at the battle of Neville's Cross, when he and the Black Rood were captured by the English. David was carted off into captivity and the Black Rood was proudly displayed at the Shrine of Saint Cuthbert at Durham Cathedral.
There the Black Rood remained as a spoil of war for the next two centuries until the time of the English Reformation, when Durham Cathedral together with most of the religious foundations in England was systematically pillaged by the agents of Henry VIII in 1540 and the Black Rood vanished without trace.
There does however, seem to be a tradition within the Sinclair family that one Simon Sinclair raided Durham and recovered the Black Rood and that it was later hidden somewhere in Rosslyn Chapel, but then a lot of things, including the Holy Grail are supposedly 'hidden away' at Rosslyn Chapel.
1 Margaret was later canonized as Saint Margaret, which only served to increase the reputation of the Black Rood
2 There is also a legend that the 'Holy Rood' was the cross that magically appeared to save David from being attacked by an angry stag.
E. Cobham Brewer The Dictionary Of Phrase And Fable (The New And Enlarged Edition of 1894)
The Catholic Encyclopedia at http://www.newadvent.org/cathen/
An article entitled Relics for all by Lord Xaviar the Eccentric
from Stefan’s Florilegium at http://www.florilegium.org
A reference to the Sinclair legend at: