Short answer
Gather Fireflies from Night flowers or Rhododendron bushes, or buy them from Gunter. Keep some for alchemy and fishing bait rather than selling every stack.
The Astrologer's Tower
View full-size image ↗Official promotional screenshot · © Lazy Bear Games / tinyBuild · Official media kit · Material credits
Gather or buy
Night flowers yield one Firefly; Rhododendron bushes can also drop them. Gunter stocks Fireflies from the start. The listed base price is 10 bronze; actual trading prices vary with stock.
Study before alchemy
A Firefly contributes 1 red and 2 blue runes. Study it first: 2 Science and 2 Faith yield 20 green and 10 blue tech points, and you keep the item. It appears in Third Eye Salt and Reanimation Liquid mixes; follow those formula guides for complete rune totals.
Turn Fireflies into Insect Bait
At a Kitchen Table, 4 Fireflies make 4 Insect Bait. Fishing III unlocks the recipe; it needs Farming mastery 4 and takes 4 ticks. Kitchen Table II also supports it.
Choose your next use
For zombie production, follow Reanimation Liquid and Resurrection. For a temporary mastery boost, follow Third Eye Salt. The official Astrologer screenshot below provides location context, rather than a Firefly spawn map.
Sources
Related pages
Official screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Night FlowerOfficial screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Third Eye SaltOfficial screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Reanimation Liquid
Official screenshot · © Lazy Bear Games / tinyBuildProgressionGraveyard Keeper 2 Resurrection