The Kings Roads

Behind every mirror and reflection, is a hidden realm of wonder. The magician king of an ancient land constructed the hidden roads using the essence of the astral sea, and it had many mysterious uses. The roads are levitating in empty space, and are of various makes and design, with occasional towers of stairs and gate houses that lead between the horizon stretching roads. The realm of the roads has no visible bottom but due to the stuff of the astral sea woven into the realm, it can be used to access most any mirror or reflective surface in the cosmos, though it is never without risk, as the pressing worlds it touch give way to accidental travellers and monsters leaking in very occasionally. Time is strange in this realm, while it is always faster to travel via the roads, there is still an incremental increase in time taken the further you travel, though it would never be as long as the time taken to physically travel from point of entry to desired location.

The caster and one creature they can touch, which requires a melee spell attack if the creature is hostile, walks into the mirror or reflective surface and into the King's Roads. To exit they must find a mirror or reflective surface of appropriate size for them to fit through. Travelling to a location the caster has been to before on the material plane is a DC 13 Intelligence (Arcana) check and a length of time decided by the DM. To travel to a place the caster has not been before on the material plane is a DC 17 Intelligence (Arcana) check. To travel to a location that the caster has been to before not on the material plane is a DC 20 Intelligence (Arcana) check and at least 3 days of travel. To travel to location unknown to the caster that is outside the material plane is a DC 23 Intelligence (Arcana) check and at least 5 days of travel. If the caster fails a check, they find themselves at the next DC check marker below the one they were attempting. If they decide to not exit the roads and push through, the caster becomes at risk of running into monsters. Whether they fail or succeed the subsequent roll, the DM rolls a d4. On a 3-4 nothing happens and the caster has either succeeded or failed again without meeting danger, on a 1-2 the DM rolls for a random encounter. 

Inspired by Susanna Clarke's 'Jonathan Strange & Mr Norrell'