It is because of it's fascinating life history that I've included it on this site - it uses subterranean moth larvae and pupae as it's host. They are also said to use buried butterfly larvae and pupae as well but I'm not aware of any butterfly larvae in the UK pupating below ground level.
LIFE CYCLE: I'm not sure if the larva ingests the fungal spores along with the grass or other foodplant it eats or whether the spores simply land on the larvae before it digs down. Either way, when the larva is fully fed it burrows down an inch or so in to the earth to pupate, the spores then germinate and begin to grow inside the host, eventually replacing the larva's insides with that of the fungi's mycelium. When mature the fungus throws out the distinctive fruitbody which emerges above ground. The swollen, dimpled upperpart of the club is the fertile area that releases the spores.
Here's one I dug up on a Halifax Scientific Society fungi foray to Gosport Clough on Oct. 20th 2018. The moth larva is still intact and fairly recognisable as such.
The above larva dissected showing the solid pack of mycelium inside.