The “Maahir’s Gambit” scenario.

I can't find the fourth artifact! Necropolis was almost defeated at the very beginning. He ran to the central isthmus using Astral, and I followed him – and he defeated me. Then I went along all the paths, collecting everything, and found 3, and destroyed the blue one. But I can't find the fourth one; I've searched the Dungeon and bought all the artifacts from the merchants. What should I do? Maybe I shouldn't have defeated him so quickly? Or does it not matter? Maybe someone can list which artifacts need to be collected?