Complex sentence: In 1321, he was elected King and crowned in 1324 in his father's lifetime and would have succeeded as sole-ruler had it not been for his premature death fighting in his father's war with Holstein.
Split into simpler sentences: In 1321, he was elected king and crowned in 1324 as his father's co-ruler. He would have succeeded as sole ruler had he not predeceased his father in 1331, fighting in his father's war with Holstein.