As soon as you come through the Portal into The Necropolis, before you go up to The Crystal Guide, on the map you will see two small blue buildings. (Look at the green arrow on the map.) The one on the left is Sylvia Drake's Tomb, and the one on the right is The Pyromancer's Tomb. You have to go down a few small steps or a small ramp to get to it. For your quest, you need The Pyromancer's Tomb.
Hope this helped.
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