Arn was born in Arnäs, Västergötland in 1150. At the age of 5, he had an accident, and was believed to be saved thanks to his mother's prayers to Saint Bernard of Clairvaux. Since the boy was saved, his parents decided that in reward for the miracle, he was to be sent to a monastery. He spent the next twelve years of his life in the monasteries Varnhem in Sweden and Vitskøl in Denmark (Vitae Scholae in Latin).
He met brother Guilbert, a former Knight Templar, who instructed him in the use of the sword and the art of medieval war in the Holy Lands. The Prior of the convent, Father Henri, told Arn to witness for himself the outside world, and only after that he would be able to take the eternal vows of poverty, chastity and obedience. Arn did so, but fell into the terrible sin of having sexual intercourse with Cecelia, who was also in the nunery. Cecelia got pregnant and told her sister Katarina about what happened between her and Arn. Katarina became jealous and told the Head Mother that not only did Cecelia have sex with Arn, but lied that Arn had had carnal knowledge before marriage with her as well. According to the laws of the church, it was a great sin to have sexual intercourse with two women who had the same mother. He was then condemmed to spend 20 years in the Holy Land, as a Knight Templar, and Cecelia was kept in the nunery and her boy child taken away from her.
At the age of 27, Arn is already a veteran in the Crusader army, when he faces the strangest experience a Knight Templar can have. While pursuing a band of Saracen thieves, he comes across the very enemy of all Christendom, Saladin, and saves his life. This idea is apparently borrowed from The Talisman by Sir Walter Scott. After this, both become very close friends, but great enemies at the same time. After a series of unimportant battles, Saladin seizes the city and fortress of Gaza, where Arn served as commander. Saladin spares Arn's life, while willing to get a bigger prize, the city of Jerusalem. Arn is given the order to march with all his knights, and they finally defeat Saladin's army at the Battle of Montgisard. When Arnaud de Toroge is named Grand Master of the Knights Templar, Arn is summoned to Jerusalem to become Master of the city.
In the Battle of Hattin, Arn is severely wounded and spends several weeks at Saladin's hospital in Damascus, after that he accompanies him to Jerusalem, which is conquered (1187). In the very end of the second book, Richard the Lionheart comes to Palestine with an army of crusaders. He captures Acre from Saladin, and when about to buy off 5,700 prisoners from the Christian, Saladin and Arn were horrified see Richard having them slaughtered, even the children "as the mamluks were riding, trying to save them, in tears." Arn then confides that Richard "only will be remembered as the slaughterer" and that he will never capture Jerusalem, thus confiding himself that he doesn't care if the city remains in Muslim hands. He embraces Saladin, and then starts traveling home to Sweden, where a state is about to be formed.
After returning to Sweden along with a group of people from the Holy Land (among them two Armenian craftsmen, two Englishmen specializing in crossbows and longbows, glass workers, felt makers, copper smiths and two learned Saracen physicians), Arn has great plans for his childhood home, Arnäs and the estate which was to be his before he was forced to the Holy Land, Forsvik. He explains his idea of "building for peace" by constructing a modern castle at Arnäs and to create a manufacturing centre at Forsvik.
Arn starts training several young men and boys to become knights (or at least a competent cavalry force) at Forsvik.
When he returns to his family they are at first overwhelmed with a joy that soon cools when he voices his desire to marry Cecilia Algotsdotter. His family wishes to use him for a political marriage, but due to the intrigues of Cecilia and her friend the queen they are married either way, earning them the enmity of Birger Brosa. After a little time they have a daughter together and Forsvik grows rapidly.
However his newfound luck is shattered when King Sverker II attempts to kill the sons of the late King Knut. Commanding his force of cavalry, he defeats the king's soldiers at the Battle of Älgarås. He also takes command at the Battle of Lena and emerges victorious.
Two years later, when King Sverker returns to once again with a Danish army and Arn commands the forces of King Erik at the Battle of Gestilren. He takes a fatal wound while pushing forward to kill King Sverker, he succeeds in this and the battle is won. He dies a few days later, now aged 60, and is buried in Varnhem monastery.
 
 
 
http://en.wikipedia.org/wiki/The_Knight_Templar_(Crusades_trilogy)
 
there is also movie about it.... watch it if you want.