The Treasure Hunters - Act 4 Side Quest

Start Location: Tsakonian Ruins
Quest Location: Tsakonian Ruins/Great Tomb of Aegimius
Objective: Speak with Herodion and Nysa

The Treasure Hunters quest begins by speaking with either Herodion or Nysa in the Tsakonian Ruins. For this quest you will need to speak with both of these NPCs, after which the quest will automatically complete. You can find Herodion in the northeastern portion of Tsakonian Ruins, just a tad northeast of the altar used in An Impossible Task. Just run up and talk to him to complete this part of the task.

Nysa is found in the Great Tomb of Aegimius which is in the northwestern most portion of the Tsakonian Ruins. My screenshot below will show you the map location of both these NPCs to help you locate them. In order for you to speak with Nysa you'll have to defeat all of the enemies in her room. Every undead with a purple name and white glowing aura around them kill and then you'll be able to interact with Nysa.

Nysa: Found in the Great Tomb of Aegimius

Herodion: Found in northeastern Tsakonian Ruins

It's worth nothing that inside of the Great Tomb of Aegimius where you find Nysa you'll also find a named skeleton, King Aegimius. He'll drop the Ancient Sun Key when you defeat him which is part of The Wealth of Ancient Kings Side Quest.