Is difficulty relative or absolute?

Thea uses skulls to indicate the difficulty of challenges, from 1 skull (easy) to 5 skulls (hard). Is this rating relative to the levels/equipment of your villagers, or absolute?

For example, suppose a fight against four level 7 monsters is rated as three skulls at the beginning of the game. A couple hundred turns in, with villagers with a lot more levels and much better gear, would that same fight still be three skulls? Or would it now show as being one skull, since it will likely be a lot easier to deal with at that point?

From experience, it's absolute. You'll notice certain same groups of enemies roving around during the entire game, and they will always have the same number of skulls.

The most noticeable is the spiders; a group of 4 spiders is always 1 Skull. A groups of 6 spiders is always 2 Skulls. Even near the start of the game, beating 6 spiders is trivial. If there was any kind of auto-correction, you should see the 6 spiders drop to 1 skull after the first dozen or so turns, but it doesn't. I've encountered them in the end-game, still having 2 skulls.

(They are also not always accurate; sometimes two groups with the same number of skills can be of vastly differing difficulty, at least in a Fight challenge; which is what you always get if the group attacks you. Maybe they are easier to handle if you attack them and are allowed to pick a different approach.)