I often write boxed text for new in-game locations. I find it easier to think through the sights, smells, and other sensations during GM prep and write them out, rather than improv’ing when I’m trying to write a game. Here’s one such location.
Deep inside the volcanic tunnels under Rissimo, the forge city, lies an ancient ruin which can open a portal into the multiverse.
On either side of the Temple of Duregar, Old Steelhand, Lord of the Forge, there are two massive bronze doors inlaid in the mountain. Behind them, carved tunnels gently spiral into its depths, inlaid with mastercraft stonework.
They lead down into a workshop area, divided up into multiple workspaces and bays. Pipes and ducts line the walls, but they are empty. An Intelligence check will reveal that it seems to be a pipeline for some slowly-flowing liquid, maybe molten lava. But there is no lava to be found, and it is about the same temperature here as it was further up.
The far end of the workshop area ends in a ledge overlooking a stony chasm, with two great, broad staircases leading into the depths. They seem much older than the rest of the workshop. In the material plane, they disappear under a lake of magma.
If the players are able to step into the Ethereal Plane (or a similar demiplane), they will discover Dhol Lodahr, the Gate of the Mountain:
At the bottom of the chasm, there is large stone disc with an archway. It is carved in fine stonework, depicting various ivies and deciduous tree leaves surrounding bark. The texture of the stone itself does not come through here in the Ethereal Plane, but each cut into the stone seems expertly and carefully made. You don’t think you’ve seen anything like it. Carved in some sort of proto-elven runes, which you’ve seen on other fey ruins before, read the words “Dhol Lodahr, the Gate of the Mountain”.
Around the disc, there are again large gulfs were you imagine magma once flowed–long before the forge city of Rissimo was built, when the tides of magma were different.
Here, there is a low, subdued thrum of magic. As you step onto the stone disc, you feel the prickle of concentrated aether, the hairs on the back of your neck stand up, and you feel a warm sensation, almost begging you to channel the power into the archway.
Any character that submits to the sensation and channels the ambient magic into the archway opens up a portal that can be used to travel between planes of the multiverse.