Complex sentence: However, it can also be a sign of ocular melanoma, extremely rare (one in 5 million people will be affected, fair skinned, blue eyed northern European) but if not untreated, will migrate to the liver and shorten life expectancy.
Split into simpler sentences: However, it can also be a sign of ocular melanoma. This condition is extremely rare (one in 5 million people will be affected--typically fair-skinned, blue-eyed northern Europeans) but if not untreated, will migrate to the liver and shorten life expectancy.