The genuine history
Henry Sinclair was born at Rosslyn Castle, near Edinburgh, in 1345. He enters Orkney history in the late 1370s - a period when the islands were without an earl.
In Orkney, the death of Earl Malise, Henry's grandfather, in 1365, had resulted in a succession dispute that was not resolved for 26 years. Henry became involved in a struggle for the possession of the earldom with Erngisl Sunesson and Alexander de Ard. Sinclair bested his two rivals, eventually being granted the earldom by the Norwegian King Haakon VI, in 1379.
His appointment was on the condition that he not only defend Orkney and Shetland but also that, if required, he provide Norway with military support. A further condition was that he build no permanent structures. This, however, was blatantly ignored. Sinclair went on to build the Kirkwall Castle shortly after.
Not much else is known about Henry Sinclair. He was killed around 1400, his demise described by the Sinclair Diploma:
"...he retirit to the parts of Orchadie and josit them to the latter tyme of his life, and deit Erile of Orchadie, and for the defence of the country was slain there cruellie by his enemiis..."
The identity of these "enemiis" and the exact circumstances of Henry's death are unclear. We do not even know where in Orkney he met his death.
What we do know is that sometime in 1401:
"The English invaded, burnt and spoiled certain islands of Orkney."
It is therefore possible that Earl Henry Sinclair died sometime in 1400, or 1401 - possibly repelling an English force who raided Orkney after the Scots attacked an English fleet outside Aberdeen.
After Henry's death, the earldom passed to his son, also called Henry. Earl Henry II was earl by name only and there is no record of him ever visiting the Orkney during his 20-year reign.
And, as far as Henry Sinclair is concerned, that is as far as recorded Orkney history goes. Now we enter the realm of myth.