The door opens into a vaulted chamber, lit by a pale blue glow that seems to radiate from the walls. The ceiling rises a good 30 feet above the floor. Supporting buttresses meet in the center, as in a cathedral's dome Alchemical equipment lines the far walls, while potion bottles, chalk, and vials litter the table closest to the center of the room. Dust coats nearly everything, from floor to ceiling.
In the exact center of the room, a magical circle is iscribed into the floor in solid gold plating. The circle seems to repel the dust that has gathered everywhere else in the laboratory, though some of the traceries of the diagram outside the circle are not as immune to the steady accumulation of the years.
Nathanael rises an eyebrow in curiosity, then steps into the vaulted chamber.
Dust rises in little clouds near his feet, revealing more gold plating on the floor. In fact, the gold inscription, hidden under the dust of years, extend far beyond the circle in the center of the laboratory - the diagram covers the entire room! The carefree entrance of the young wizard has triggered the etching's magic.
A warm wind hurls the dust up into the air, where it sparkles and shines in the room's blue glow. The hazy nimbus surrounds you, sweeping you up in a surge of magical force, and the room spins around and around in a blur until your eyes feel as though they will fly from your head and a whirring fills your ears and suddenly -
The room stops spinning. No, it's not the room anymore. You're lying supine wihtin a glowing lattice. And you feel hot.
Avernus, first layer of Baator
You have a moment of disorientation while your head clears from the magical transit. The stench of sulfur and ozone fills your nostrils. Hard stone supports your weight. The glowing blue lattice has faded from your sight, allowing you to notice that you're lying atop a circular inscribed diagram - an exact duplicate of the one that brought you here, but black instead of gold.
From off to one side comes a sharp intake of breath. Turning, you see the fiend.
Staring slack-jawed at you is a spiny little critter about three feet tall with wings and a spiked tail, carrying a military fork. It is backing into the gloom of a nearby crevice in a stony wall, keeping a close eye on you all the while.