How the necromancer destroyed the world. (Maybe)

The Necromancer had a special 9th level spell that he created called "Undead Power Link." That spell allowed him to also tap into the power of UNdead bodies to power his spells. Since the undead have a link to the negative material plane, the Necromancer basically had a way of stealing that Negative material plane energy from the undead. With enough undead around him, he could cast any spell he wished WITHOUT losing it from memory and NOT suffering any side effects of the spell (like ageing from Wishes or Haste, or Con loss from Permanency).

It was the Necromancer's use of Undead Power Link that caused the destruction of the world. The Necromancer went to the site of a war. There were thousands on each side. He caused many of the dead to rise as undead Wraiths. The Wraiths joined the war by killing as many as possible and causing those dead to become Wraiths. When the battle was done there were over 6,000 Wraiths surrounding the Necromancer.

He cast his spell Undead Power Link and siphoned off all their energy in repeated wishes for god-dom. The Wishes caused a portal to open to the Positive Material plane. The Undead Power Link, with all those Wraiths, caused a portal to open to the Negative Material Plane. When the powers collided, the world was destroyed.