Skull of God (eruption)

Half a day's travel in every direction from the skull, in sheltered valleys and on high plateaus, stand "the penitents" - closely spaced pillars of soft, dusty stone ranging from two to sixteen feet in height. People avoid them because of their vaguely humanoid shape (and because they make poor construction materials). Small birds, covered in like-colored dust for camouflage, nest among them. No other beasts dare approach.

Halfway up a plateau no one has ever climbed sits a monk, meditating, concerned that the tremors are imperceptibly escalating. The monk will relocate to a ridge only two hours hike from the skull for more direct observation.

The Monk: I had some initial ideas about who this monk should be, but after my talk last time of killing the sun and the moon, it's hard not to go with a monk (hah, that mouseover text) of the sun.

The Birds: I like these birds as the iridescent embers of a failed wish spell, ineffectively pursuing their task in perpetuity, but I also want them to be pieces of the "penitents'" souls so as to imply that a legendary Phoenix is a piece of the Skull's soul.

I'm of two minds about the terrain immediately around the skull. On the one hand, if it's flat and frozen, then the eruption could shatter it and make new hills and a lake where there were none before; on the other hand, having rivers and ridges already there would give the players some choice of where to weather the event. Plus ships would get stranded upriver, bridges would be knocked out downriver, and the river could run backwards for a ways. 

[The eruption is a great image, but it's more of an interruption than an adventure. Haven't worked out the larger scenario to drop it into yet.]


Eruption

A skeletal arm a mile long will burst out of the ground next to the skull, devastating miles and miles like the lateral blast from Mount Saint Helens. My draft timeline:

• Two years (or 1d6 years) ago there was a prophecy that something major would happen at the skull in the next couple decades.
• Two weeks now of small earthquakes, growing to three per hour by the end. Avalanches and blocked roads become regular threats. Animals act subdued and anxious before each quake.
• Steam explodes from the ground about 1000 ft away from the skull, blasting a crater on the side away from the grove.
• For six weeks afterwards,
• steam explosions start once per hour in the surrounding area, becoming ever smaller and less frequent.
• the crater grows by a foot per day while the ground in front of it rises by a foot per day. 
• the priestesses largely retreat into their grove, and warn everyone else to leave the city; a few of the longest residents ignore them, some of the newest regard the whole thing as a ploy, and the most adventurous attempt to scale to the skull's upper socket or dig into the growing pit.
• anyone sensitive to mystic omens will notice a series of increasingly dire warnings, although the days immediately prior to the event are relatively quiet. 
• Day of the blast, turn 1: earthquake, noticed by anyone whose Passive Perception exceeds their distance (in miles) from the skull.
• Turn 2-3: anyone with a view to the skull will see four bone-white spires rise a couple hundred feet out of the pit as they shove a mile of earth forward. This dark, roiling wall of rock and mud and ash eats all sound, and suppresses divination, teleportation and communication magic as though it were the Underdark. It advances one mile per minute (over five hundred feet per turn) through a sixteen mile cone. 
• Turns 4-7: the monastic observer casts Sending and reports these events to the PCs, pauses, then closes with "it's going to get me, too." No response thereafter. Anyone who can see the cloud approaching will see red lightning streak from the ash cloud to the ground.
• Minutes 1-8: the blast hugs the ground for the first eight miles, shredding everything and hurling the debris forward, leaving bare earth and shattered stumps behind.
• Minutes 9-16: trunks (stripped of branches) are blown down instead of carried off, the blastwave begins to swirl, separate and merge chaotically. Some people survive in the sheltered lees of steep hills and ridges.  
• The last 0 to 2.5 miles: trees are scorched but not blown down. The blast rises off the ground, leaving a stark (sometimes vertical, sometimes angled) line between scorched and healthy trees. 
• Minutes 17-32: The deepest or most directly affected parts of the cone can be buried up to six hundred feet deep, and everywhere in the affected zone gets at least a few inches of ash. If there's lava (or some kind of necrotic or fungal goo?) too, it may cut the sacred grove off from the surrounding forest.
• Several hours later: the cloud causes complete darkness in a city two hundred and fifty miles (over a week's walk) away. 
• Three days later: the cloud has crossed the continent, and disrupts seasonal weather systems on the other side. 

At whatever mile the blast reaches the party:

• Turns 1-2: gusting wind blows campfires and hair horizontal.
• Turn 3: thick ash reduces visibility to one foot and imposes an eerie silence and stillness. 
• Turn ?: everyone and everything is knocked down as searing heat (immediate and longer near the river or in the open, delayed and shorter in the lee of a steep ridge) sizzles hair and burns flesh, but not cloth or paper. DEX Save to get into cover negating half the damage (area, bludgeoning), CON save to negate the other half (atmospheric, heat). 
• Two minutes later: the sky clears but rocks continue to fall.
• Less than a minute later: darkness returns with a thick ash fall (visibility reduced to one foot over the next ten minutes, then slowly clears over the next few hours); ground too hot to walk barefoot, muddy ground can burn you, pitch boiling out of trees can burn you. No open water cool and clean enough to drink. Short Rests impossible in this atmosphere.

The DC for all saving throws is 30 minus the distance to the skull in miles, with advantage if you're in the lee of a steep hill or ridge, in a sealed vehicle, in a hole, or buried under thick logs or other substantial cover, and disadvantage if you're on a ridge with a view of the skull, or near the river. 10d6 damage? Needs to be high enough that the monastic observer is for sure dead at eight miles, but not so high that unlucky PCs can't survive farther out.

Any character killed by the blast must make a CHA Save, or their body petrifies and their soul is expelled in the form of an iridescent bird.

...Anyone suffering burn damage should probably make periodic saves against heat exhaustion afterwards.

If you're on the river: a lahar follows behind the blast at about 1/3rd its speed, felling trees 30ft up either bank and carrying the wreck of at least one bridge.

• Miles 17-32, or outside the cone: no damage from the blast, but the sun still vanishes and oppressive heat is still felt. The water turns into muddy rapids, with logs of increasing size, two minutes before the lahar arrives. 
• Anyone swept up in the lahar is subject to both crushing and drowning if they don't get balanced atop a log. Call it a DC 15 Acrobatics or Athletic check, where however many points you failed by is the number of rounds you fail for before you can try again? And you take 1d6 damage every ten rounds you're in the water?
• Then a DC 15 INT + Acrobatics or Athletics check to try getting out where the water is shallow and safe-ish?
• Mile 50: the lahar reaches its greatest size.

Aftermath

The skeleton's hand comes to rest palm-down at the mouth of its newly-excavated trench. The downward slope exposes the arm to the elbow, with the rest covered in loose, churned-up earth. The skull has tilted several degrees. Quakes peter out, and the skeleton shows no signs of moving again.

Search and rescue is rough when you don't have helicopters. What do the pilgrims and grove have in the way of rescue parties? If someone had been alternating vigils with the sun monk, the sun monk could've given them his escape plan (a way to summon a giant eagle?) which could be used to search for survivors.

The blast zone would've had loggers, fishers etc. whose livelihoods require them to be there, plus pilgrims squatting in abandoned cabins, hiking closer for a better look at the eruption, or injured or trapped while fleeing. Low roads could easily be buried in a hundred feet of debris. Navigating the forest afterwards would be a challenge: perhaps a Survival Check, where different levels of success indicate inverse levels of difficulty on a subsequent Athletics, Nature, or Animal Handling obstacle, with the option to avoid the obstacle and retry for a more favorable Survival Check but at the cost of CON saves against continuing heat or choking dust.


PROBLEMS

A bunch of the survivor stories from Mount Saint Helens involve people jumping in their cars in the first minute and vamoosing at highway speeds. D&D characters are basically limited to sheltering in place, though, and I feel weird choosing their level of danger for them.

Because the blast zone is so big, I think I'd rig the journey so by default the PCs would camp overnight in the middle of the survivable zone, then ask what time of day they want to arrive at the skull as a prompt for them to change it. I'd also try to give different campsites different advantages:

• Camping with other pilgrims lets you gather information (and the pilgrims' map of forest roads would give the PCs advantage on finding safe roads after the eruption).
• Camping near the river gives potential access to faster travel (and the possibility of hearing the blast before it arrives, but also a worse blast). 
• Camping on top of a ridge gives you a long view ahead, and everyone else a long view of you (and the best possible warning for the eruption, but also the worst positioning for the blast). 
• Camping next to a cliff or steep hill is the most tactically defensible, but least convenient for scouting or resuming travel (and best protected from the blast).

In general, I think the better protected the PCs are from the blast, the harder it'll be to hear it coming.

Why is the party in the blast zone anyways? I originally envisioned an army getting wiped out as they marched on the skull to try and stop whatever as-yet-unknown thing the earthquakes were building up to; but the PCs would most likely be ranging ahead (unknowingly into the guaranteed death zone) or getting out of Dodge (they might knowingly lure an army there, but that's not the kind of thing I would count on). If the PCs know that the eruption is going to happen, maybe they have also learned a method for delaying the eruption, aiming it or triggering it early which requires them to be in the blast zone and for which different positions in the zone have different advantages. Alternatively, I suppose they could be trying to extract a key person from the blast zone, or trying to locate and dig up an artifact ahead of the blast.

If instead it's the invader who is somehow evoking this catastrophic blast, the skeleton could be raising its hand as a protective shield.

Putting the skull in the tundra out in the middle of nowhere makes for great imagery, but also makes it hard to get a hostile army out there. I suppose there could be a second skeleton concealed in a hill or mountain somewhere closer to civilization.

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The [birds = penitents' souls] thing is borrowed from a small OSR blog I read years back and am having no luck googling. I think they got the idea from a PBS special on the Andes Mountains, with birds nesting in the side of glaciers.