This is just a drop-in replacement for Will-o'-the-Wisps updated, as suggested by its previous owner. I will deprecate this mod again when the original mod has been updated for Factorio 0.18.

Update:
As Will-o'-the-Wisps updated has been updated to Factorio 1.0, there is no reason to keep this mod alive. Contrary to what I wrote above, I won't deprecate it because it is still useful with Factorio 0.18. I just won't update it anymore -- unless it should be needed again in the future.

Please note: Development of this mod has ceased. You can still use it with Factorio 0.18, but version 0.18.7 (which is labeled as compatible with Factorio 1.0) will just redirect you to the original mod!

The following text has been copied from "Will-o'-the-Wisps updated":

Compatibility update and tweaks to Will-o'-the-wisps mod by Betep3akata.
[Feedback forum thread link] - [github link]

Important: This mod is reported to cause desync issues in multiplayer games.
I try to fix known causes of this, and feel free to report any, but never play mp myself,
so don't test changes there or know much about how it works beyond other people's reports.

Description

Adds enigmatic Will-o'-the-Wisps to the game, which can be seen around forests at night.

One current goal of the mod is to add some visible consequences to destroying alien forests or dumping pollution into them, but not necessarily damaging ones (see mod options).
Another one is making alien nature look more alive and mysterious, as nights in particular can look a bit dull there.
Third optional one is to add some light wisp-deterrent base-planning element, though still not sure how much.

New alien fauna:

• Yellow Will-o'-Wisp
  - They live in forests and aren't generally aggressive.
  - Destruction of trees at night time will draw them out in droves.
• Red Will-o'-Wisp
  - Rare wisps that live under rocks.
  - Physical damage will split them into multiple ones.
• Green Will-o'-the-Wisp
  - Smaller harmless will-o-wisps that are faster and more curious than others.
• Purple Will-o'-Wisp
  - Will-o'-the-Wisp spore.
  - Cause corrosion of structures and power production infrastructure in particular.

Technologies:

• UV Lamp (solar energy research) - only known way to reduce lifespan of purple Will-o'-Wisps when it's dark.
• Wisp Detector (alien biotech research) - outputs number of wisps around it to circuit network with a range input.
• Pet Will-o'-Wisp Lantern (combat robots) - lasting UV-resistant wisp drone to use as a personal lantern.

Hints

• Avoid burning forests at night.
• Trees destroyed or mined at night-time might spawn yellow wisps, rocks - red ones.
• Forests can be hard to find on a desert map, fix via moisture/trees bias in map generator.
• Purple wisps are only dangerous to specific building categories, like power production, refineries and defences.
• Wisps in the area attack when one of them is killed, but are not very fast or dangerous in low numbers.
• Disable turrets targeting wisps in Mod Options menu to avoid accidental aggression.
• UV will chase wisps away without any retaliation. Overlapping lamps will amplify effects.
• Circuit control wires can switch UV lamps on/off for power saving reasons - they eat quite a lot.
• Wisp detectors range can be set by R signal (on itself or via wires), see ingame description for more info.
• Lantern will-o-wisps last a long time, but can always be put down (C key) if they're a bother.
• Wisps appear in forests after dark (in a day or few), drawn out by nearby pollution and player activity.
• Mod only tracks surface darkness changes, not time, so mods that change day/night cycle should be compatible.
• There are mod options to control where, which and how many will-o'-the-wisps can be seen on the map.
• Set "Wisp aggression factor" in mod options to e.g. 0.5 or something higher for a more challenging game.
• See forum thread for more technical info and debug commands.

Changes from the original mod

• Compatibility update, various bugfixes.
• Mod Options menu settings for wisps/turrets aggression and spawning.
• Different spawning algorithm based on pollution.
• Rebalanced stats, resistances, costs of technologies.
• Some new technologies, visual tweaks.
• Ingame changelog with full list of features, fixes and tweaks for each version.

See original Will-o'-the-wisps mod and its forum thread for more information.

Cosmetic Compatibility Mod(s)

• Inlaid UV-lamp - makes UV lamps look like ones in Schall Lamp Contrast and Inlaid Lamps Extended mods.

Mod Suggestions

• Longer Days and Nights - more nights, more shinies!
• Time Tools - UI clock and speed settings plus time/darkness-tracking combinators.
• Diplomacy and Rampant - for emergent game faction dynamics and challenge.