The Nightmare Forces

The remains of Lorelei's raw magic, which was taken from her after she betrayed Alidia

Chaos, Dark

Dark, smoky grey, murky

Magic enchantment, physical changes

Magical enhancement
Body enhancement (makes the host bigger/taller)

Holds memories from Lorelei

Horn Rot (if pony does not become a host to it)
Drastic changes in appearance, severe damage to psyche

Mania, psychopathy,

Evil

Feared

Darkness/shadows

Heal,

Decays what it comes in contact with, for the most part. It needs a host, so it searches for one. It will only take the most worthy host, and discards lower beings (such as small animals, or creatures that have less power than ponies) after decaying them completely. But beings with more power, it will try to corrupt, in order to build up more power.
Has some of Lorelei's knowledge and memories, which will be passed down to hosts. Nothing super clear, just little things like what alicorns are.