Sandstorm (D&D 5th Edition Environmental Hazard)

Michael O. Varhola
Environmental hazards of various sorts are something we made a point of including on the various encounter tables that appear in the "Swords of Kos Fantasy Campaign Setting" (and in the separate prequel volume "Encounters"), and sandstorms are among the possibilities in desert environments in general and places like Aigyptos and Mesopotamia in particular. Because the Kos campaign setting is universal and not keyed to any particular role-playing game system, those environmental hazards do not have any specific game crunch associated with them. Now that we are moving ahead with a number of 5th Edition projects tying in with the Kos milieu, such as my current project, "Aigyptos: A Gazeteer for 5th Edition," we are modeling such effects in game terms, and this is one of the more impressive results. 
Environmental Hazard: Sandstorm

While sand and dust are frequently blowing around the deserts and semi-arid farmlands of Aigyptos, a true sandstorm can rise up in the wake of otherwise regular bad weather, on its own, or as the result of potent magic, and can be quite dangerous. Most people familiar with these powerful phenomena know that the best response to an incoming sandstorm is to seek protection from the best cover possible and wait until it is over (options that might not always be available or viable to adventurers). A typical sandstorm will be a mile high and might be as much as 60 miles wide and stretch from one end of the horizon to another and will last 2d12 hours.

Characters can attempt to make Wisdom (Perception) or Wisdom (Survival) ability checks, whichever is better, to spot an incoming sandstorm, and how well they do will determine how far out it is when they notice it. 
Roll             Time to Sandstorm
30                5 minutes
25                4 minutes
20                3 minutes
15                2 minutes
1-14             Number of rounds equal to roll
Characters unfamiliar with sandstorms might not understand what they are looking at or know the best means for dealing with it. Those experienced with such phenomena, however, might be able to use the time they have bought to seek appropriate cover or take other actions to prepare themselves for what is coming. 
Wind strength within a typical sandstorm will be 20 to 60 (2d3x10) miles per hour and visibility is reduced to 10 to 60 (1d6x10) feet. Within areas unprotected from the storm all ranged weapon attacks are at disadvantage; gas and vapors are immediately dispersed; candles, torches, and similar unprotect flames are extinguished; and protected flames, such as those of lanterns, will dance wildly and have a 50% chance per minute of being blown out. Furthermore, each foot of movement costs two feet of speed, and this penalty stacks with that for difficult terrain, causing each foot of movement to cost three feet of speed for characters exposed to both sandstorms and things like jagged lava fields.
For each hour or portion thereof that a character is exposed to a sandstorm it must make a DC 16 Constitution check and a DC 16 Dexterity check.
      * If a creature fails the Constitution check it suffers 1d10 points of slashing damage, mostly to its respiratory system and lungs, and if it fails on a roll of 1 then it also acquires one level of exhaustion and must make another DC 16 saving throw to avoid contracting a random disease.
      * If the creature fails its Dexterity check then it is blinded until such time as this condition is reversed by appropriate magic (e.g., Lesser Restoration) or it subjected to three successful daily DC 16 Wisdom (Medicine) checks. It three of these daily checks are successful the creature will have its sight restored and if three fail before this happens then it will remain blinded until this condition can be reversed magically. None of these successes or failure need to be consecutive, just to total three.
Both the Constitution and Dexterity saves are made at advantage for characters sheltering in areas like huts, unsealed ruins, shallow open caves, and similar places, and are unnecessary for characters in places like shuttered stone buildings, closed up tombs, and the like. Characters attempting to shelter under cloaks, cover their faces, and employ similar measures can opt to make Wisdom (Survival) rolls at the same DCs in lieu of either sort of saving throw, and someone with this skill can extend its benefits to a number of companions equal to its proficiency bonus.