Eric XIV of Sweden
Eric XIV (December 13th, 1533 - February 26th, 1577) reigned as King of Sweden from 1560 until he was deposed in 1568.
While he has been regarded as intelligent and artistically skilled, as well as politically ambitious, early in his reign he showed signs of mental instability, a condition that eventually led to insanity. He displayed paranoid behavior for most of his life and was particularly hostile to the Swedish nobility, which led to him arresting his own half-brother, John.
Despite this, Eric was also highly ambitious and made great strides to expand Sweden's influence. He was capable as a commander and repelled multiple invaders during a number of wars that occurred during his reign.
Eric, having been deposed and imprisoned, was most likely murdered. An examination of his remains in 1958 confirmed that he probably died of arsenic poisoning.