As the group enters the northeast passage:
This passage, some 10 feet wide and about as high, ends abruptly. The last 10 feet or so of the floor and ceiling are loose earth, rather than worked stone. The ceiling has a hole, some 8 or so feet in diameter, stretching upward beyond the radius of your light.
The Centipede Swarm lurks amongst the soil at the top of the tunnel and is not initially visible. It does not engage until someone attempts to climb past it. (See below.)
Features of the Area
The tunnel upward: The passage stretches up 50 feet to the The Fields. The walls are crumbling earth, making the climb slow and dangerous (treat as uneven, slippery surface, base DC 20 for Athletics checks to climb.) Furthermore, the soil at the top is home to a lone Centipede Swarm, which hides (Perception DC 19 to notice), waiting until an unfortunate climber is within easy striking range.
Engraving: There is something faint scratched into the dead-end wall, though the PCs may not notice without searching carefully.
Perception Check (DC 20):
Something is scratched faintly in the rough-hewn dead-end wall:
How I long to feel the fresh earth beneath my feet,
the scent of the morning dew in the air,
and the heat of the fire that warms my crops
The message mentions three of the four elements. The missing element, water, can provide the PCs with assistance climbing the tunnel.
The Magic Seed: Lying amidst the loose earth of the floor is a tiny magic seed. Again, without searching, the PCs are unlikely to notice it.
Perception Check (DC 25):
Lying atop the loose earth of the floor, directly beneath the center of the overhead passage, is a small black seed.
If detected, the seed shows powerful alteration magic.
If the players water the seed while it rests on the soil beneath the tunnel (regardless of whether they’ve even noticed it):
Rapidly, the tiny black seed swells in size and cracks, sprouting roots and a tremendous black stalk. The crypt shakes as it grows rapidly upward through the tunnel, as far as the eye can see. Then, just as suddenly, its growth halts.
The gigantic black beanstalk forms a makeshift ladder, much easier to climb than the crumbling walls (treat as a slippery ladder, base DC 5 for Athletics checks to climb). Further, it enables them to completely avoid the Centipede Swarm lurking in the tunnel walls above, and instead proceed directly to The Fields on climbing out.