A Druthonae is a kind of latent schism in the fabric of reality. Places where the boundaries of different realities, times, and places unexpectedly meet. Normally they are not 'active', that is the barriers between realities are intact and these kind of 'holes' in space-time are not visible, nor do they cause any ill effects. There is a natural resistance to one point bleeding into the other, it takes a large burst of energy directed at the weakest point in the barrier to open the schism and make it active. When the same points in space, time, and realities touch, it forms a bridge between the two points until the barriers close back up.
Destination
A single Druthonae connects two points, sometimes the connection flows just one way but most often the connection flows in both directions. Even if the flow of matter / energy can flow in both directions a Druthonae connects two points of space, time, and reality to each other and that is it. A Druthonae cannot be changed, moved, or altered by any known means to change the origin or destination.
Time Travel
The connection is relative and moves as time moves. So if a Druthonae allows passage to the past (or future) the difference in time remains constant. So walk from 1990 to 1925 then in 2025 you could walk from there into 1960 along a specific path of reality. Temporal Druthonae tend to be symmetrical, that is to say matter / energy can flow in both directions.
Reality
A Druthonae can connect any reality (i.e. subspace domain) and any universe (i.e. timeline) and always connects the same two. Passages far flung realities with a high Codivergance Factor (the factor of difference of physical laws / properties from one reality to another) tend to not be symmetrical (matter / energy can only flow one direction).
Magic, Psionics, and Technology
A Druthonae can be opened in a variety of ways, broken into two basic categories. Opening with Aether (psionics, magic, artifact, etc.) or opening with technology.
Triggering with Aether
The most effective way to open a Druthonae is to use a large burst of magical or psionic energy to trigger the barriers to temporarily align and the schism open into a bridge. Spells to accomplish this must find the sweet-spot where all the barriers are most closely aligned, then push them together -- this is usually done as a single spell and requires the least amount of effort and energy.
Triggering with Technology
Using technology to open a Druthonae is also possible, it requires the direction of a high powered subspace beam calibrated to also hit at the point of least resistance between the subspace barriers. Once the subspace energy beam hits the barriers, a modulated graviton pulse can trigger the alignment and open the bridge.
Unexplained Quirks
The Druthonae anomalies also hold some peculiar and unexplained properties.
Distribution
Druthonae are not evenly distributed across space-time as one might expect, they tend to be most prevalent on planets with rocky cores. Some do exist in space but are mostly present in deep space, not near any other phenomena. Most Druthonae also tend to be in open places such as deep space, an open field, or the sky. The reason for this peculiar distribution pattern is unknown.
Atmospherically Safe
There is a natural resistance for energy / matter passing through the bridge even when open. This gives the very strange property that if one were to open a bridge to a planet with a toxic atmosphere or to hard vacuum, the bridge itself is resistant to allowing the atmosphere to pass through it. Similarly if one opens a bridge too in the ocean, inside a star, or too close to a black hole the atmosphere will not be sucked out or the ocean / star interior won't cross the bridge. No one is quite sure how this happens. Some energy will bleed through (i.e. a bridge to the interior of a star would get hot, but not so hot as a vaporize a planet -- as would likely happen simply bridging the surface of a planet to the interior of a star.
Origin
There is usually a kind of void between different points in time or reality that are not supposed to touch. These voids stop something from crossing from one moment to some place in the past or even from one reality to another reality. In some places this void is thinner and one reality or time can directly touch another moment in another reality or bridge two times in the same reality together. There is no known cause and it is believed that these are woven into the very fabric of reality and are thus naturally occurring. It is theorized that allowing matter and energy to pass between realities may be a naturally occurring way for a given reality to avoid eventual entropic failure.
Myths and Legends
Some myths would indicate that these bridges could explain how some figures of folklore could travel back and forth between places and even time periods regularly without being seen. Ancient legends indicate the Aos sí and some related mythical figures may have used these throughout ancient Earth. Other figures throughout Earth's own history had abilities which could be explained by use of these bridges. Similar legends exist on many worlds, potentially indicating the use of these bridges in pre-history of many worlds.
Connection to the Verum Transitus
The Verum Transitus makes use of these bridges, allowing a person to pass magically through the multiverse from one point to any other point. Like a car traveling a freeway system with infinite exists. Each door within the Transitus bridges across different Druthonae without triggering them to activate allowing a the traveler to bet swept through the multiverse relaying between Druthonae without the Transitus having to punch directly from one place to another.