Manual of the Planes
David Knott
The 4E Manual of the Planes is very useful as a DM reference but less useful for players, as the only materials useful for them are the planar paragon paths and various rituals for planehopping. For the DM, this book is loaded with material for extraplanar adventures as well as notes for adapting the cosmology for an individual campaign, with the Great Wheel used as an example. Compared to the Great Wheel, the Mystaran cosmology seems easier to adapt - it actually has fewer differences from the 4E standard.
A minor note of interest is the mention of the Isle of Dread, which this book has moving around between the normal world(s) and the Feywild.
In an interview with the authors, one item they mentioned deleting deliberately was the Astral Projection spell, instead focusing on portals to move between planes. This change fits in quite well with the published Master level adventures, among others.
In converting the Mystara cosmology to 4E, one decision that needs to be made is what to do with the Ethereal Plane. While one could follow the guidelines in the Great Wheel sidebar, a more straightforward approach would be to leave the Elemental Chaos in its place and treat the four elemental planes as the regions in the Elemental Chaos with the greatest concentrations of each element.
The Astral Plane can be replaced by the Astral Sea with no diffficulty, and the various outer planes dropped in with little or no change. The Entropic plane of Pyts corresponds to the Abyss, which in the Mystara cosmology replaces the natural world as the plane with direct connections to both the Astral Sea and the Elemental Chaos. The only practical effects of severing the direct connection between the natural world and the Astral Sea are the addition of an intermediate stop in the Elemental Chaos when plane shifting between the two, the requirement for creatures of immortal origin to have an affiliation with a particular element (Sphere) to interact with the natural world, and the resulting removal of devils as monsters likely to be found on or near Mystara.
The alternate dimensions of Myth and Nightmare can be reached by traveling vast distances across the Astral Sea. Because of the great similarities between the natural worlds of Myth and Mystara, gates between the two are relatively easy to establish. The Nightmare Dimension would borrow characteristics from both the Far Realm and the Plane of Dreams in addition to its own unique quirks. The Astral Plane description in Wrath of the Immortals applies to the region of the Astral Sea near the Nightmare Dimension. Somewhere beyond the Nightmare Dimension can be found the Vortex Dimension that terrifies even the Immortals.
The conflict between gods and Primordials is reflected in the conflict between the Sphere of Entropy and the other four Spheres, with some interesting complications from some immortal creatures serving Entropy and some elemental creatures opposing it.
David Knott
Now that 4E has been out for a while, I thought I would toss out a few ideas
for combining Mystara with 4E:
1. Which books to use? With the ready availability and great usefulness of
WotC's Character Builder, I would be inclined to say make use of whatever
they support. There are now enough races, classes, and other material
available to provide a decent approximation of just about any Mystaran
character type, with very little of that material going to waste.
2. For a Mystara DM, my primary notes would be to make sparing use of
creatures with the Aberrant origin and not to use creatures with the
Immortal origin at all, with minor exceptions reskinned to different
origins.
3. The idea that player characters are special takes care of any unheard of
character types such as dwarf wizards. I would probably make a note that
historically there have been no dwarves who were arcane spellcasters. If a
player wants to be the first dwarf wizard, he is welcome to -- but it is not
as though that would be as popular a combination in 4E as it was in 3E.
So -- does anyone else want to summarize their conversion work so far? I
recall seeing lots of monsters in the Mystara 4E section of Piazza before
the move.