Izual NPC

Izual's spirit.

Izual is a monster when your character first sights him. He must be defeated and destroyed to complete the Fallen Angel quest. Once he's been defeated his spirit floats up from his corpse and hovers there long enough to deliver a grim speech:

Tyrael was a fool to have trusted me!

You see, it was I who told Diablo and his Brothers about the Soulstones and how to corrupt them. It was I who helped the Prime Evils mastermind their own exile to your world.

The plan we set in motion so long ago cannot be stopped by any mortal agency. Hell, itself, is poised to spill forth into your world like a tidal wave of blood and nightmares.

You and all your kind . . . are doomed.

His speech completed, Izual fades away, leaving your character to deliver these glad tidings to Tyrael, who takes them about as well as can be expected.

Lore

"The greatest of these heroes was Izual, lieutenant to the Arch-Angel Tyrael and bearer of the Angelic Runeblade Azurewrath. He once led a fierce attack upon the Hellforge as the creation of the dark demonblade Shadowfang was nearing completion. His quest was to destroy both wielder and weapon- a charge that he was destined never to complete. Izual was overcome by the legions of chaos and, tragically, was lost to the Darkness. His fate stands as testament to the fact that Angels and Demons alike shall fearlessly enter into any domain - so long as their hated enemies dwell within."
-Diablo 1 Manual

Most histories of the Great Conflict like the above describe Izual as a misguided martyr-hero. Even Tyrael always believed his most trusted Lieutenant was a brave fool. However, it is revealed in Act IV of Diablo II that Izual did not succumb to hubris, but rather joined with the Prime Evils in plotting their own exile on Sanctuary - Dark Exile. In his speech during Act IV, Izual said that their plan could "not be stopped by any mortal agency", and since the player fails to stop Baal corrupting the Worldstone later it is not certain whether that plan was foiled. However, it was clear from his twisted appearance and the fact he had lost the angelic blade Azurewrath that not everything had gone well even before Izual's death, so his speech could have been bravado.