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I looked at Wikipedia's list of emperors to no avail. I realized he may have been a king of Bohemia, so I went back and looked at that. It was John the Blind, son of Henry VII, Holy Roman Emperor. The Luxemburgs lost the crown to the Louis IV Wittelsbach. Henry therefore was only king of Bohemia and Poland, but never emperor. I was going to sack my question, but I highly recommend reading about this fellow.
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You may be referring to Don Juan of Austria. He never became Holy Roman Emperor because he was the illegitimate son of Emperor Charles V, in the sixteenth century.
But "son" of the Emperor helped get him into high military positions, where his abilities allowed him to have many adventures, and to make a name for himself, notably at the (naval) Battle of Lepanto against the Turks.