First, the plant is very dessicated. It hasn't been taking up enough water. That is either because you haven't watered enough, or the roots are in bad shape.
The spot is most likely sunburn or heat burn. Was it close to a window? A candle? The stove, or a heater?
Be sure the roots are OK. If you need to it is fine to pull a Phal. out of the pot to examine the roots. Doing so won't harm the plant.
__________________ May the bridges I've burned light my way.
I agree with es. The black spot (unless it grows in size or more show up) isn’t your biggest concern right now, the dehydration and root health would be my priority. Definitely take a look at the roots.