Portal of the Elder Brain (Patreon)

The House of the Ten Chambers of Errestos is an arcane cult dedicated to unraveling mysteries mathematical and metamagical. Their membership is small and scattered throughout many places of learning both arcane and divine. The largest group of the Castellans of Ten Chambers live and work in the city of Kaldath where they have painstakingly assembled a physical reproduction of some of the interdimensional mathematics that they study.

The residents of the city have other names for the Ten Chambers of Esterros, generally referring to the cult members as "the brains" and their metamathematical temple-fortress as "the Portal of the Elder Brain" based on the rumours that the brains of the six eldest of the order have their brains pickled and built into the structure itself to power a failed gateway to another world.

The gateway, however, was not a failure (even if the fanciful stories of the elders of the house's brains powering it are wrong). The House of the Ten Chambers of Errestos contains a dormant portal just offset from the centre of the structure that is powered by the mathematical and semi-spiritual processes that occur in the other nine chambers of the building. Through intense meditation, working on fractal sand drawings, re-arranging stone puzzles, and other acts in the chambers, the portal can be opened to the various elemental planes as well as some other places in this world and others.

Of course, when a strange cult has a strange home and engages in activities beyond the ken of normal folk, it doesn't take long for even stranger rumours to take hold. Who knows what strange intelligences the order has managed to contact via their portal? And who is to say that these intelligences haven't in turn taken over the order and now threaten the city that plays host to it? Doesn't the term "elder brain" immediately evoke terror of the tentacled monsters of the far realms? Surely there is an abomination here that must be discovered and excised by brave adventurers!

For a 5e D&D game, the Castellans of the House of the Ten Chambers of Errestos would make a perfect order of monks of the Way of the Four Elements - practicing perfection mathematical, physical, spiritual and elemental. 

Generally when I draw a map, I have a pretty strong aversion to symmetrical design. I drew this map (as well as the Tomb of the 14 Imperial Princesses) in an effort to allow for some more symmetry in my work because players expect at least some symmetry because of the general positive feeling that pattern recognition provides in most people (including those who design these dungeons and other environments in the game world).

Thanks to YOUR awesome support, this map is available for free personal or commercial use. Download the high resolution map from the blog post at http://wp.me/psUVp-2s5