Poison sources
Natural poison sources keep their surroundings permanently toxic.
The visible poisonous growths of the Undead Grove are fixed environmental sources. Their effect forms a lasting toxic ground where Undead can meet their particular need. They themselves remain immune. A slime without immunity first accumulates exposure there and is poisoned only after sufficiently long or repeated contact.
Source or trace
In the toxicity view, natural sources appear alongside residue from decaying carcasses and Undead that have lingered in one place. The similar-looking areas have different lifespans: a passing trace fades and washes away faster in rain; a fixed growth does not. This distinction matters to the enduring sustenance of Undead.
A lasting danger
A source is neither harvested nor carried as a parcel. It changes the surrounding area instead. Brief contact usually leaves other slimes below the poisoning threshold; away from toxic tiles, their exposure falls again. Nearby sources can add together, however, and standing still or fighting in poison lengthens exposure. Entrances and their immediate surroundings remain clear when the world is built, so no growth seals a nest.
An Undead rested in the middle of the turquoise growth. The Aqua crossed its edge quickly, but greenish bubbles rose between two sources.