The white-tipped thread-like fruit bodies on old beech mast in leaf litter are the sterile or asexual state of this fungus.  Initially black near the sterile base and whitish with conidia (asexual spores) towards the tips, the whole of the fruitbody eventually blackens and becomes covered in minute pimples as the spores mature.  The pimples are the openings of the spore-producing perithecia inside the fruitbody. 

Photograph on beech mast; note substrate.

Old beech capsules buried in leaf litter.

All year round.

https://www.first-nature.com/fungi/xylaria-carpophila.php