It had always been an intention of Bruce's to crusade but affairs within his own country had precluded this. So on his deathbed, on 7th June 1329, had Douglas make a promise that once he, the Bruce, was dead his heart would be removed from his body and taken by Douglas on crusade against the infidels in the holy land. A silver casket was constructed to hold the Bruce's heart and in early 1330, accompanied by a number of knights and men-at-arms, Douglas set sail from Berwick. Douglas's company sailed by way of Sluys then Santander and then down the coast of Portugal. Knowing that the moors were attempting to increase their foothold in Europe, they sailed up the Guadalquivir to Seville. Once there they offered their services to Alfonso XI of Castile, who had a base at Seville.
Douglas, due to his past exploits, was one of the most famous men in Europe. Understandably many knights including many of his old adversary's, the English, gathered to see this renowned warrior. Disbelief travelled through the gathered knights at sight of Douglas, this could not be the great warrior, he bore not the slightest scar upon his face. This was a time when most knights would have borne scars attesting to their former battles. Douglas remarked "I have always had strong hands to protect my face".
The Scots were assimilated into Alfonso's army, Douglas being given command of a portion. A move eastward was then made to besiege the moors who had recently captured a fortress midway between Granada and Seville, the Castillo della Estrella (Castle of the Star), close by the small village of Teba.
The Christian army settled in for the siege but a Moorish army led by Osmin, a notable soldier, came to the relief of the castle and encamped at Turron on the other side of the Rio Guadalteba. Osmin attempted to deceive the Christians by sending one part of his force toward the castle, the main force circling around in order to attack the Christian camp. Alfonso realising what was happening sent a force including Douglas and his men down to contest the river crossing, keeping the bulk of his forces back to protect the camp.
It is not exactly clear what happened next, suffice to say that Douglas and his men found themselves deep into enemy territory. Forming his men into a wedge formation, himself at the front, they charged at the Moorish forces in an attempt to drive through and back to the main body of the Christian army. Almost on the point of breaking through to safety, Douglas seeing William Sinclair of Roslin isolated and attacked by the moors pulled his steed around and headed back into the melee. The moors quickly closed in around them and Douglas disappeared under the surge of moors.
Osmin meanwhile finding the Christian camp well defended had withdrawn from the field of battle. In the aftermath of the battle a Scot, one William Keith of Galston, who had earlier suffered a broken arm and had took no part in the current fight, took to searching the battlefield with the remaining Scots. They found Douglas's body, with five fatal wounds, surrounded by many bodies of the enemy. On retrieving Douglas's body the silver casket containing Bruce's heart was discovered underneath him. A few days later the castle fell to the Christian forces.
Douglas's men would not hear of him being buried on foreign soil, so in line with current practices his body was boiled in a cauldron until the flesh fell from the bones, This was buried in hallowed ground at Teba, his skeleton and heart were taken back to Scotland and interred in St Brides church in Douglas village. Bruce's heart was conveyed to Melrose Abbey and there buried.