Complex sentence: Swedish king John III and Stefan Batory allied against Ivan IV in December 1577, despite problems caused by the death of Sigimund which meant that the issue of the substantial inheritance due to John's wife, Catherine had not been resolved.
Split into simpler sentences: Swedish king John III and Stefan Batory allied against Ivan IV in December 1577, despite problems. The death of Sigimund had meant John's wife, Catherine, stood for a substantial inheritance which had not been resolved.