Although this autumn has been very mild, the air is still scented by the rotting breakdown of the previous season's abundant growth. That means mushrooms, those marvellous and mysterious things. The US poet Marvin Bell wrote: "Each mushroom was a button, each a flowering, some glow in the dark ... The dead man has seen them take the shapes of cups and saucers, of sponges, logs and bird nests."

Up on Halkyn Mountain, we find bird's nest fungi (Crucibulum laeve) in a churchyard so old the church isn't even there any more. The curious growths on dead twigs are in fact round, leathery cups, each containing four or more cream, lentil-like "eggs" - containers of spores that are ejected when hit by a single raindrop.

We continue our walk through the deathly damp drizzle on to ancient grassland. On the undisturbed sward, where the mycorrhizal networks have been allowed to crisscross through the soil for thousands of years, we find some real gems, including more scarce species. There's a host of waxcaps, tiny and delicate, and coloured orange, yellow and red; and moss caps, some smelling of cedar wood or honey. We find dewdrop mottlegill - slimy brown and holding on to tiny mist droplets under its pointed hood. Then there are funnel caps, bark bonnets, scurfy twiglet, and candle snuff, which appears like black teeth with white crowns. Weirdest of all is the earth fingers (or dead man's fingers) fungus - a black hand reaching out from underground.

But the star of the show arrives as a red spike - a zombie caterpillar fungus (Cordyceps militaris). This scarce, gothic thing reprogrammes insect brains to climb higher, before the fruiting body of the fungus bursts out of the dead insect and releases its fungal spores. This inspired the game, and then the TV series, The Last of Us, and on the BBC's Planet Earth an ant was shown infested with Cordyceps.

There are hundreds of species of Cordyceps worldwide, with at least a dozen in Britain. C militaris is said to have a range of hosts, including ghost moth, which we have trapped before on Halkyn Mountain. In Tibet, Cordyceps are collected for medicine, and are now being investigated here for treatment of some fatal diseases. So all this death leads to regeneration - how magical.

* Under the Changing Skies: The Best of the Guardian's Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 15% discount