(First, a note to those of you who may have known other worlds in the past: although it shares the name of another cosmological region, the Astral Sea in this Converged world is no different than the so-called Elemental Chaos below. Their substances mingle here, no doubt much to the chagrin of the elementals and astral beings and whatnot.)
The Astral Sea is the vast, engulfing swell of energy surrounding all worlds and all things. The astronomers of Erff had always assumed that the space around their world was more or less empty of substance, but this is hardly true; the Astral Sea is every bit as liquid as any terrestrial body of water, but composed of raw elemental flows and transient, magic infused proto-matter. Dark matter, indeed.
The majority of the sea is a free-flowing conglomerate of raw elemental forces of all imaginable kinds. However, flows of more rarified energies – necrotic, radiant, etc. – flash through in unchartable and largely unpredictable currents, stirring the elemental protoplasm and keeping things moving. From whence these currents originate and what destination they hurtle towards is unfathomable to mortals.
The sea is predictably deadly to mortals, for there are no known creatures able to breathe magic, and whatever dark matter is available is almost certainly not configured in the proper form. That is not to say that the Astral Sea is devoid of life, however. Spirit beings are capable of navigating the Sea with a minimum of discomfort, but even they will soon flag and tire against the constant pressure and churning. Mortal wizards have developed a means to project their spirits outside of their bodies, enabling temporary Astral exploration, though this is fraught with danger that true spirits do not experience.
There are two ways of traversing the Astral Sea safely. One is to use a portal. Portals form when one of the many flagellating strands of reality thrown off by worlds has actually latched onto something else in it’s frame of reference. Although the conditions for portals being open or visible vary, they are generally safe and reliable. (Do note that this has no bearing on the entry and exit points; a portal from your backyard to the heart of a volcano on the plane next door will safely and reliably transfer molten rock as easy as it will you.)
The other way is by use of of an Astral Shell, the most basic form of portable realspace. An Astral Shell is, in essence, the tip of one of a plane’s free-roving portal strands, sheared off and prepared for mortal (or immortal) habitation. It is a tiny, self-propelled bubble of reality that can be used to slowly corkscrew through the Sea. The secrets of their creation have only been divined by arch-conjurors on a mere handful of worlds in the infinite Sea, so don’t go expecting to get your hands on one easily. That said, they are almost indestructible so long as they never fully re-enter a world, which would subsume their reality membrane into that world’s. Some worlds in the great unknown have begun improving Astral Shells into more fully-fledged vehicles, some of which are large and populated enough to be kin to worlds, but these are rare indeed. It is whispered that the Gith, both Githyanki and Githzerai, are among these; the two species are almost uniquely at home in the Astral Sea, even though it can easily be as deadly to them as any other mortal race. (The two races are also almost entirely unknown in the Converged world, existing in legend only. Gith are usually smart enough to avoid planes that are currently undergoing massive reality distortions.)
There are dark tales that whisper of creatures truly native to the Sea, fearsome unknowable tentacled monstrosities that have as little understanding and respect for life as mortals do for the Astral Sea’s inner workings. Some mad warlocks claim to have even contacted these beasts, but, frankly, the rantings of mad would-be mages who haven’t even the patience to learn a single cantrip and must instead wheedle them away from spirit beings is hardly confirmation of the phenomenon.