Lucid Dreaming To Stop Dream Invaders
I doubt if the example of sigil has any spells over it.
Magic on Earth is Reigniting. We have moved into the age of Aquarius where anything is possible. Yes we had magic before but now it’s more accessible and more powerful than before. Hence there are alot of Succubus and Incubus travelling the sleep realm. (solutions below)
You are the one in powered to control your dreams. Nightmares are often due to dream walkers who pay sorcerers for certain medallions or Sigil, magic symbol with elite spiritual Siphon symbols. With the right enchantment they quote the spell and have power to delve & control the narrative of random sleeping dreamers.