Partly updated for Enshrouded 1.0— 1.0: the unlock cost; the rest is pre-1.0 wiki data

Inner Fires
Maximum Shroud Time increased by 2 minutes, allowing you to explore for longer. Shroud Time Should this time run out, the Shroud will consume you.
Where Inner Fires sits on the 1.0 tree
In the Core area. The cheapest way in from a starting point buys 2 nodes for 2 points, this one included (one rank each).
Builds that use Inner Fires
1 preset build tagged INNER_FIRES — defining picks first, then ranked by tier (S → A → B).
Related Ranger skills
Same archetype — likely candidates to slot next to Inner Fires in a build.
Airborne1pValues in the 1.0 files: 12% per rank, 3 ranks.
Arachnoid1pValues in the 1.0 files: 20% per rank, 3 ranks.
Backstab Mastery1pValues in the 1.0 files: 20% per rank, 3 ranks.
Blessed Arrows1pValues in the 1.0 files: 8 per rank, 3 ranks.
Counter Battery1pValues in the 1.0 files: 8% per rank, 3 ranks.
Dexterity1pIncreases your Dexterity attribute by 1 Increases Bow and Dagger damage by 5% per Attribute Point
Eagles Bane1pDamage against flying enemies increased by 30%.
Endurance1pIncreases your Endurance attribute by 1 Increases Stamina by 10 per Attribute Point
Sources: game files 1.0 + Keen Games 1.0 patch notes (changelog, combat changes) for the unlock cost; enshrouded.wiki.gg (pre-1.0) for the description and effects.