I've written about songlines before, back when I was using Openquest to run the Dawnlands, but as I convert it over to Mythras, it's time for new rules.
A brief recap of songlines for those new to the Dawnlands:
Outside of the cities of Durona and Kaddish, there are no maps of the Dawnlands, and even in the cities, most maps are cadastral surveys used to sort out parcels of land rather than tools of navigation. While people are loosely familiar with the concept of maps, the low levels of literacy in the hinterlands would make them near useless anyhow.
Instead, people use songlines to get around. These are songs that encode the necessary directions for someone to get from one place mentioned in the song to another place mentioned in the song by decoding the rhythm, tempo, mode, scansion and actual semantic content of the lyrics.
Most songlines are vast historical epics tracing the journeys of heroes and peoples across the Dawnlands, criss-crossing and entangling one another to create both a tight mesh of navigational information as well as a comprehensive history. Songlines do not necessarily trace the shortest distance between two points it may connect, and part of the expertise and lore of using them well is to understand when and how to switch from one songline to another to cut a journey short. The elders of a clan typically serve as a storehouse of knowledge about the songlines, and clans trade unfamiliar or new songlines with one another as prized goods.
What this means is that there are two skills in Mythras that allow one to find one's way from place to place. The first is the Sing skill, and the second is the Navigate skill. Navigate covers overland travel off the songlines (which for the purposes of the skill's description on pg. 48 of Mythras are "unusual journeys" "in completely unfamiliar territory"). It works exactly as described in the Mythras corebook, and is mainly used by people who learn it as a professional skill through their careers.
When PCs are following songlines, which count as the "normal" way to travel in the Dawnlands, they use Sing to find their way (Sing is a standard skill available to all characters).
To determine the length of a journey, either pick a number between 50 and 1000, or roll 1d1000. This is the percentage the navigator has to accumulate in an extended task roll using their Sing skill in order to successfully guide the party to where it wants to go.
Each day of travel, they roll their Sing skill. On a critical success they accumulate 50%, on a standard success 25%, on a failure 0%, and on a fumble, -25%. When they have accumulated a percentage roll equal to or higher than the roll of 1d1000, they have arrived at their destination. If for any reason they drop below 0 due to fumbles, they are lost. It's very hard to get lost while using songlines, but they also channel travellers along courses that may not be the most direct route, and other travellers, bandits, etc. are much more likely to be following songlines themselves rather than roaming around randomly.
PCs may aid one another or augment their Sing skills with relevant passions, skills, etc. If they can collect an especially useful or direct songline, they can shift the base difficulty of the Sing rolls down to Easy (rolling 1.5x their skill).
For every 100% accumulated, the PCs will come across a landmark or area of interest that serves to reorient them with a new verse (that is, verses typically cover 2-4 days worth of travel).
PCs can also use the rules for crafting equipment on pgs. 65-67 of the Mythras corebook for crafting songlines, using the Sing skill for task rolls. This requires them to have travelled the course involved, and can either involve merging together two or more songlines, or being part of a group where someone successfully uses the Navigate skill to find the way.