I finally got to see Greed's realm!
On very rare occasions, when you kill a Treasure Goblin, they open a portal to Greed's realm, which is an area called "The Vault". It's a relatively small area with piles of gold and some Treasure Goblins, and an end area called the "Inner Sanctum" which contains a boss called the
Baroness of Greed. One thing to note is, the portal to The Vault can only be accessed by players who were in the game at the time the portal opened. (Sorry Throbble!)
At the end I had found ~3 mil gold, 13 Marquise Gems of various types, 15 Imperial Gems of various types, a legendary crusader shield, a legendary ring, and a legendary gem that only drops from Greed herself.
Here are screenshots of the start, end, and gem:
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