From the Feywilds to a New World

Manten, Lord of the Mistveil forest, after being peeved by the party, attempts to send them back to their homeworld. However, as they don't know the name of their world, he sends them to "The First World": the first world to ever have been created in the material plane. 
In a blink, the players find themselves on the side of a mountain path, standing in front of the maw of a cave. They are located in Valendroh, in the world of Plaxus. Overlooking a rocky plain filled with dead trees, sand, and cracked earth, there are no signs of life save for scattered cloths.
Perception DC 12: The cloths are tied into the rough shape of an arrow, pointing towards a distant blur on the horizon. Perception/investigation DC 17: The distant blur appears to be in the rough shape of a squared off pyramid.
In the cave is an old, terrifyingly old Dragonborn lich. Almost just bones at this point, the only signs of life are that his head is able to move slightly, and his eyes glow with a dull green. This is Trellis, the last king of Valendroh. He is able to tell the players that the beast has no real name, but came to be known as "The Hollow", and that it is an abomination, as it is an Avatar of a god with no god to control it.        "While it is not eldritch by any means, the Hollow was once the vessel for a god to interact with the Material Plane. However, the god itself was unpopular and was considered evil, and gradually, its name was not allowed to be spoken. Belief didn't stop overnight, of course. But with time, adventurers and authorities destroyed cults, burned all texts that even mentioned its name, and eventually committed an ultimate taboo. One party, calling themselves "heroes" on a quest to finally destroy the evil god, sacrificed themselves to contain the god within a prison where it could not influence the world to renew belief in itself. For a while, there was peace. For generations. Until the efforts paid off, the last person who believed died without telling anyone, and the god's power dwindled to nothing. On that day, an awful crack split the earth, and from it emerged a beast of shifting form. Like wild magic, it was unpredictable, unintelligent, violent, and destructive. It lashed out as it moved, wrecking havoc everywhere it went, chasing any life in order to siphon their inherent power, from life and from magic. My people...We did our best to help them escape. My advisors and I opened a portal into another world to evacuate the population, to hopefully live better lives. It was all we could do. Mapan, Pascal, Blute...I hope they are doing well. It is all I can do to watch over the entrance now."         If further questioned, it is revealed that he used Message to send word to all of his servants in the First World, otherwise known as "Illum", that The Hollow appeared to be attempting to replicate the magic of the portal to chase after the life it sensed on the other side.
Wherever the party travels, there are small, rough signs pointing towards the distant blur, which is a Mayan-style pyramid, covered in dust and debris, eroded after years and years of wind. The pyramid has small puzzles, that are much to basic to keep anyone with a brain out:

At the base of the smooth ramp leading up is a lever with slots next to it, almost like a simple maze, which needs to be maneuvered into the other side. Once pushed there, the slats constructing the ramp shift to become stairs.

At the door at the top of the pyramid are a series of slots on the wall, with shaped tiles on the floor. The slots and tiles are shaped as a star, a diamond, a rhombus, a triangle, and a circle. All need to be placed in their corresponding hole to allow the door to open.

Inside the pyramid, there is a squared-off spiral staircase leading down into the earth, with a bright yellow light at the bottom. However, there is a missing section of stairs at the very top. A small needle sticks out of the wall. DC 14 Per/Inv: the floor in front of the needle has dried blood on it. Once a player pricks their finger on the needle, the missing section of stairs comes out of the wall and allows passage. 

On the way down the stairs, the players are assaulted by The Hollow, attempting to bash its way through the magic barriers of the pyramid. Once combat starts, it will have a special stat block. The players' goal will be to reach the portal at the bottom before dying. The portal is 250 feet of stairs down, or 180 feet straight down.
Upon escaping through the portal, the players find themselves at the top of a near identical pyramid, this time in pristine condition, as they start the main plot "Home Sweet Home...ish"