The Slaver Fungus
What is an aggressive ecology? It's a new type of challenge for your players¸ a monster with many faces¸ a web of interrelated challenges that can together form a complex encounter-in-depth. It is a world-building device based upon a magical interrelationship between disparate¸ yet connected creatures. It is¸ in short¸ something not seen before and something not quickly forgotten.
Most aggressive ecologies are of a terrestrial nature¸ indigenous to a single planet¸ and found only in a particular location. But some few travel between the planets¸ hitchhiking on space-faring matter that eventually lands upon a new and unwitting location. The slaver fungus one such aggressive ecology that arrives on a new world via a meteoroid strike¸ and if conditions are favorable it expands outward from that location¸ feeding upon the local organic material until it grows large enough to reproduce¸ sending its exploding spores to the very limits of the atmosphere. In that instant¸ it becomes a new threat throughout the entire world.
This aggressive ecology presents a terrible challenge to all 1e/OSRIC Player Characters and is easily adaptable to any OSR system. Present your players a new type of challenge¸ the Aggressive Ecology!