This is my subjective list of most important/relevant to Tibian lore NPCs.
1. King Tibianus III 
King Tibianus III is the sixth king of the Thaian kingdom, after succeeding his father Xenom. He is a direct descendant of Banor, the first human. King Tibianus has the power to promote worthy adventurers. He claims to buy the Excalibug, though its existence is unproven. His dog Noodles is known as the best sniffer dog in the kingdom.
The Thaian Kingdom is the largest kingdom in Tibia. Territories under his rule: Thais, Fibula, Greenshore, Venore (de facto sovereign), Edron (including Stonehome), Cormaya, Liberty Bay and the rest of Vandura, Port Hope, Rookgaard (including Tutorial Island and the Island of Destiny), Meluna, Travora
Books: The Cities of the Continent
2. Henricus 
Henricus is the Lord Inquisitioner, which means that he leads the Inquisition. He sells the Blessing of the Inquisition after completing The Inquisition Quest, in which he is main NPC.
The Inquisition is the organization that is entrusted by 'all churches of the gods' to do their holy work, meaning to clean Tibia of unholy beings like Witches, Demons and Vampires.
The things the holy Inquisition do include killing Vampire Lords and Minions of the Ruthless Seven and helping others with the same cause. The Lord Inquisitioner is named Henricus, who holds office in Thais. Recently another base has been established on the island of Roshamuul, under guidance of Sandomo.
3. Avar Tar 
He is a Hero on a quest for King Tibianus. He has information about the Excalibug, the Ruthless Seven and the Pits of Inferno.
We can also assume he speaks Bonelord's language 469 - he can tell us a poem:
Avar Tar: 29639 46781! 9063376290 3222011 677 80322429 67538 14805394, 6880326 677 63378129 337011 72683 149630 4378! 453 639 578300 986372 2953639! I know it’s rather short, but still, this poem I like best.
4. A Prisoner 
The prisoner is also known as the Mad Mage. Before becoming a prisoner he lived elsewhere in the cave system in the so-called Mad Mage Room.
He was locked up in a tower in the Mintwallin prison because Markwin could not understand him. There is a note about him writen by Markwin in one of his books. You will need Key 3620 to open the door to the prison he is held in.
5. Arkhothep 
Arkhothep is the dark pharaoh that rules the undead city of Ankrahmun. The pharaoh is both the high priest and the primary deity of an obscure cult that celebrates the undead as the key to immortality.
Books: Mahatheb the Visier * Morgathla Reveals the Secrets of the Mask * The Great Pyramid and the Undying Pharaoh * The Lair of Xemal Xoth
Honorable mentions:
https://tibia.fandom.com/wiki/Queen_Eloise
https://tibia.fandom.com/wiki/Emperor_Kruzak
https://tibia.fandom.com/wiki/Markwin
https://tibia.fandom.com/wiki/Eremo
https://tibia.fandom.com/wiki/Zoltan
https://tibia.fandom.com/wiki/Basilisk
https://tibia.fandom.com/wiki/Lugri
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