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In the chaotic underground of Skullport, a city of thieves, murderers and slave traders, the Skulls imposed some semblance of order when things get out of hand. They are magically powerful floating skulls believed to be the spiritual embodiment of the arcanists who created the Mythallar. The Mythallar or mantle is a sentient and powerful arcane relic capable of generating nearly limitless power. The Mythallar in Skullport was created to support the immense weight of the world surrounding it, but at some point it seems that the magic had faltered, and Skullport was in danger of complete destruction. Only through the efforts of the arcanists did the caverns survive but at the cost of their own lives. The thirteen most powerful were reborn shortly after as the Skulls, guardians of the mantle.
They however were bound to the mantle and can't move more than 300 feet beyond. In centuries they lost their individual identities and started act as a collective hive mind. Each Skull retained few aspect of his original identity but in objective and purpose they were really a single entity. Halaster Blackcloak, when discovered the ruins of Sargauth Enclave and the Skulls, decided to avoid them and also prohibited to his apprentices to enter the area. Only Shradin Mulophor decided to ignore the warning and with the reluctant benediction of Halaster reclaim the ruins as for his own and even made a pact with the Skulls. Unknown to all the Skulls agreed to Shradin's request to create a free port not for boredom or whimsy but as part of a plan to escape from the bound to the mythallar. They tried to absorb in the mantle chosen spellcasters to replace them. A first success was in 1358 during the Time of Troubles when Halaster's control over Undermountain's wards wavered and the Skulls were able to transform the Rag Mage in the Fourteenth Skull. Again during the Halaster's Higharvestide of 1369 the Skulls subvert Shradin and repeat over him the process. So they discovered that of all fifteen entities bound to the mantle two were able to move outside the bound.
They so understand that need only to link to the mantle eleven more spellcasters but their plan was ruined in 1373 by Vhostym that for his own plot tapped in the mythal with a powerful artifact destroying four Skulls and transforming the remaining in true Undead creatures, now again all bound to the mantle.