I never played dnd before 5.0 , but red undead was always supposed to be super scary in older editions. At least I'm as a player used to the fact that low level undead in 5.0 are dumb easy.
The ghost one is two Wraith with hover speed of 60 (which I nerfed to 30), Life Drain attack and a huge list of damage resistances and some immunity on top of fair amount of HP. Plus ability to Create Specter from humanoid that has been dead for no longer than 1 minute and died violently.
Most terrible aspect though were my high rolls, I'm afraid. I'm really happy it is just one PC dead and not a TPK here.
So let's take a long rest and proceed from there, shall we?