Direction Ability in an Underdark Hexcrawl

Many years ago, I ran the 5e adventure Out of the Abyss, which takes place mostly in the Underdark. I felt like one of the weakest parts of that adventure was modeling travel from one location to another. At the time, I had no clue what a hexcrawl was, and so I basically glossed over the travel bits. Now that I've learned about hexcrawling though, I've been thinking about how I would run Underdark exploration if I were to do it today.

We could come up with a few novel terrain types (mushroom forests, volcanic tunnels, tar pits, etc.) and then run the Underdark effectively the same as a surface hex crawl. I think that would work, but it's missing something. As Justin Alexander points out in his own blog post about underdark hexcrawls, the Underdark is unique in that travel from one hex to another is not always possible. 

Passages leading from one hex to another can appear and disappear suddenly due to cave-ins, monsters burrowing, and tectonic motions. A more immersive hexcrawl would have a mechanic representing that navigational difficulty; and I think one way of doing that is by stealing some rules from the game Outdoor Survival by Avalon Hill. 

Each Outdoor Survival scenario includes a chart, called the "Direction Ability" chart, defining how a token is allowed to move across the board in a given turn. Depending on the result of a d6, the player's movement may be randomized, forced into a straight line, or otherwise limited in some way.

For example, in the Search scenario, the Direction ability chart has a 2-in-6 chance to require using all your movement in a straight line, a 3-in-6 chance to require using all your movement with only one direction change, and a 1-in-6 chance of allowing free, unstructured movement

The Direction Ability chart for the "Search" scenario

I think that this system could work very well for the Underdark. You can create your own Direction Ability chart (or steal the ones from Outdoor Survival) and have the players roll for the direction ability at the start of the day. Then run the hexcrawl as you would normally. The movement restrictions provided by the chart represent the various blockages, delays, and inconsistencies you encounter from traveling through such a dangerous place.

You could expand on this idea by creating different charts for different regions, or by having random encounters that reward players with more movement options, like shortcuts or extra direction changes. You can imagine a surprise attack from a purple worm or umber hulk creating a tunnel that the party can move down. 

Perhaps large settlements would have Underdark guides who the party can hire. These guides could interaction with the direction ability in interesting ways as long as they are in the party's employ. Maybe they could modify the Direction Ability roll toward more favorable movement results. Or maybe they would give the players extra directional changes when they would usually wouldn't get any. Of course, the guide would make an excellent target for wandering monsters, if you were so inclined.

If I get another chance to run an exploration in the Underdark, I'll likely start by stealing the Direction Ability chart from the Search scenario, and evolve it from there based on how that game goes.