A web of mycelium that permeates the ground and constantly regenerates itself, occasionally producing visible fruiting bodies.
It's "immortal" only in the sense that an organism with distinct genetics doesn't die of old age, but fungi aren't really individual organisms like we are.

It looks like dirt. Or, depending on your perspective, a forest

How does it work? Imagine nanobots created to control nature. It connects to all the plants, creating little tubes to exchange nutrients and electrical messages between them, in exchange for a nutrient "tax". Split the network in half, and now you have two. Put them back together, sometimes even entirely different species of mycelium, and you have one.

How do they reproduce? All the ways. They range from 2-8 distinct stages of lifecycle. Sometimes they have haploid reproduction, sometimes they recombine their own genetics, sometimes they clone themselves. Sometimes they have more than 2 parents.

Sometimes they have extra special forms like truffles that only come out in certain conditions. Sometimes they have multiple variants of mushrooms with the same genetics. Sometimes they possess multiple distinct sets of genetics

Mushrooms are just the sexual organs of the mycelium... Sometimes they spread based on time, or based on moisture, or just when they feel like it. Sometimes they don't have mushrooms at all

Mycillium does everything in every way, their spores can literally call down rain and they choose what plants live and die. It looks like they have language based on analysis of the electrical signals running through them.

The more you talk about them, the more insane you sound