The setting is clearly inspired by Planescape: it focuses on a huge city, Ys, that contains portals to other planes of existence. But it's not simply a Planescape clone: Ys has its own flavour (among other things, it's an imperial centre, which controls territories on other worlds).
The setting looks intriguing and fun:
At the centre of the game is the city of Ys, a baroque fantasy city which rules territories on three different worlds. The worlds Ys is in contact with vary in nature, with some being Earth-like, others less so.
Magic is common and an accepted part of Ysian society. The key reason for Ys's success is its army and powerful magical guild working together in territories outside the city.
These territories include the ocean world of Tethys, with its traders, undersea monsters, pirates and religious zealots. Then there is Marn, a world of many small human kingdoms, intelligent humanoid lizards, and vast reptilian monsters. Most recent of the Ysian conquests is Samara, a world of warring kingdoms and a strict caste system.
There are stranger places. Ys has a presence on the blasted desert world of Nespo, with its war machines and undead. At the fringes of Ysian knowledge is the Burning Heaven, a place which may be literally heaven or hell. And connecting them all are the endless tunnels of the world of Erebus.
But the heart of Ys is rotten. Its overlord, the Autarch, is literally a monster. His lieutenants, the Agents of the Autarch, sow discord in the city. The Autarch leaves the apparatus of government and the rule of law to crumble and divides the other power structures in the city, ensuring that nobody can depose him.
The player characters are members of the Explorer's Guild, hired by powerful Patrons to step through one of the magical portals into one of these worlds and undertake adventures on their behalf.
More information on both the setting and system (including a "preview" PDF) is available here.
Anyhow, I'm backing this and am looking forward to reading it. It looks like it would be easy to combine with my (as yet unused) Planescape materials, among other things. (And D101 has produced some very fine games.)