Complex sentence: Barad-dûr was destroyed for the final time on T.A.25th March 3019 when Gollum took the Ring from Frodo and fell into the Cracks of Doom, the Ring was unmade and Sauron was defeated.
Split into simpler sentences: Barad-dûr was destroyed for the final time on T.A.25th March 3019 when the Ring was unmade. Gollum seized the Ring and fell into the Cracks of Doom, the Ring was destroyed and Sauron was defeated.