Enough about monsters, let’s talk places.
There’s the world, obviously. It’s where we all walk around and have our adventures. I assume it’s a sphere, but haven’t had to argue the point yet. It’s got a sun and a moon; let’s assume we’re the center of it all, and that beyond the veil of the stars (a cloak with pinpricks of light, but finite) there is a great boiling sea of azoth, raw creation.
There’s a few parallel worlds, lying on the obverse of the natural one. The most accessible of them is the Nether, a place of shadow, mist, reflection, and twilight. It is a place of entropy and hunger; holding close to the world in the nether is difficult, as unseen tides pull you away from the shore into the Deep Nether, which is an infinite and hungry sea. It’s a place of the dead, and the tides pull them deeper. Undead, resistant to the tide and intrusive to the world, upset this order and channel the living to resist it. The Deep Nether is sometimes called Lethe, a place of forgetfulness and sleep, where souls dwindle away.
In places where the nether has been twisted with magic and made thin, attempts to enter it lead instead to an entire second world behind our own: Gehenna, a toxic hellscape overrun with aberrations, lies just behind the world. Its sun is red, its land is poison, its core is a mazy twist of deadly dungeons which enclose ancient terrors. Slaad, gibbering mouthers, mind flayers, beholders, gith of both types, mimics, aboleth. Its native plant life is a venemous hybrid with snakes; its jungles are overrun with snake people: yuan-ti, naga, marilith.
The elves have a series of more bounded worlds associated with them: the intricate shadow world of Fymory, and the twilit groves of Illyria. They’re not very different from the point of view of reaching them; they’re a patchwork of locally continuous, non-locally shifting terrain: like clouds moving across the sky, they come into and out of conjunction with each other at the edges.
In all three cases — Gehenna, Fymory and Illyria — they can be reached by “crossings”, areas of intermittent magical area. If the crossing isn’t active, an attempt to reach the nether will usually activate it; if it is already active, it is as easy as walking to reach stranger skies.
There are other conjunctive demiplanes, of course. Haunted houses likely attach to an area of the nether so warped that it might as well be a demiplane, and tales persist of constellations of mirrors linked by some event which permit far-travel.
There are then two more fundamental planes to discuss: Azoth, and the Astral. Azoth is the plane of magic, the plane of possibilities, the plane of madness. An ever-shifting place of energy and rippling cause-and-effect, it is the source of magic, of demons, of elementals, and where many demi-planes bubble. Exposure to raw azoth has a warping effect, but if mediated by a demiplane — a realm of azoth made relatable albeit still raw — it can instead be a place of wonder or terror.
The Astral Temple is a much more austere realm, where the “machine elf” modrons and the self-appointed Silver Lodge of gith, gnomes and other astral wanderers keep the mechanisms of the cosmos functioning. The Astral Temple is a place of force and light hovering in an infinite black abyss below and an infinite blasting sun above, timeless and omnipresent. It is difficult to reach without potent spells; Plane Shift cannot reach it, but Astral Travel can. It is coterminous with every point in the world and its shadows, at every point in time. Time travel, however, invites paradox, which is enormously destructive to the fabric of the temple.
Beyond that: who knows?