Erik XIV is the first Swedish king to inherit the title when daddy Gustav Vasa died in 1560. Erik was then on his way to propose to Elisabeth of England, but had to cancel the trip. The plan to marry Elisabeth was not just a whim, but part of Erik's rather grand and clever foreign policy - trying to make the Baltic Sea a Swedish Mare Nostrum and gaining land in the west to also reach the North sea coast. (What is today west Sweden was then Denmark.) Not much of that happened during Erik's reign, but the policy for the future was set. The period from 1560 to 1721 is often called the "long 17th century" or the Great power era in Swedish history.
Erik was intermittently mad, and when sane he was still extremely hot tempered and prone to make rather stupid descisions, such as marrying and crowning a young barmaid, Karin Månsdotter, as queen and denying compensation to families who's members he had killed in a fit of rage. In 1568 he was deposed and his younger brother Johan III became king. Johan held Erik imprisoned until 1577, when he had him killed by arsenik.
