The only advice one can offer, is not to attempt your first ascent of the mountain in darkness.
Nobody should traverse totally new terrain, at the dead of night, even with a head torch, there is every possibility that mist may come down, and obscure the way.
A traverse of Pen Y Fan in daylight hours, is very straightforward in the right weather conditions, but one has the security of daytime to make a hasty retreat if the weather or visibility turns nasty.
Most years, i venture to the summit of Snowdon for New Year, but have turned back several times, simply because the weather further up the mountain proved challenging, and never having had any walking related accidents, i know when to turn around.
I know Snowdon and Pen Y Fan very well, ive visited both their summits so many times, ive lost count, but making an attempt on a mountain that is nearly 3000ft in altitude, having never been to its summit, at the dead of night.
Its not advisable.