You need to get Gerald in Castle Keep in the Library. He will ask you to get his Bestiary. (You can find Gold Monster Statues or pay 2k gold to complete bestiaries). Make sure you complete Castle Keep to get him in your Artisan's Workshop.
a. You will want to get lockpicking on your toon and go back to the beginning of the map. Go up on the left side of the grand corridor and on the 2nd left, enter that room and move down one room, go left another room, and in next room you will see a LP door where you will find the bestiary. 2. Gerald will now offer you a pattern to make the Dimensional key (Once created it will be in your artifact page and you will need to add it in your loadout to have it on a map).

To make the key, you will need to get the following:
2 Dimensional Shards (you get them from Forgotten Tomb (100% drop rate on destruction of the portal or you can get them from Guardian's Gate but if your RNG is like mine, just join someone to finish FT ha)
10 of each of the following: Sapphire, Emerald, Amethyst, and Topaz. (These are created by Mobrick and you just appraise whatever gem color you need, you get the gems from Sunken Caverns by having a miner mine them. Have Brumik make the Obsidian Pickaxe to get better quality gems btw)
Once your key is created, put it in your loadout and head into the Zookeeper Mission in the dropdown.

In the Zookeeper mission (Abandoned Zoo from Gerald), from the start, head up to the top right of the map. Make sure you kill stuff to get your key artifact. You will come up to a "space or alien" looking area of the map. Eventually you will run into a little tent/hut and a broken lever with a teleportation platform. Here is where your key will work.
Go inside and you will see Jonas! Grab him, go back to other dimension, and head back to the portal to complete the map

Now you will see Jonas in the Artisan's Workshop with some new quests.

Jonas offers the following to start with:
Pet Food: 800 Gold, 1 lizardman eye (Caverns or Sunken Caverns), 1 Darkling Eye (Caverns, Sunken Caverns), 1 Dust Devil Eye (Cursed Sands or Forgotten Tomb), 1 Watcher Eye (Sewer or Infested Sewer or Arena)
Common Lure: 200 gold, 3 Scorpion Stinger (), 1 Slime Core (Sewers or Infested Sewer)
Rare Lure: 200 gold, 3 Darkling Eye (Caverns, Sunken Caverns), 1 Slime Core (Sewers or Infested Sewer)
Epic Lure: 200 gold, 3 Lizardman Eye (Caverns or Sunken Caverns), 1 Slime Core (Sewers or Infested Sewer)

Happy Hunting!