Nightmares of X-Men Now
The movements from the realm of darkness were intriguing, to say the least. Deals and movement and pacts galore. Which meant, to be prepared, Nightmare needed some additional power. However, moving on the more mystically inclined elements was too dangerous. He'd been defeated by Strange and the godly forces too often.
Searching, however, he found a mass well of psychic energy with few magical wards, with many powerful minds, some of them psychic, some of them with related potential, ripe for the taking.

Shifting to the dream realm reflection of Westchester, deeper than the few wards began, he shifted and shaped the realm, extending vague fingerling tunnels into the dreams of the inhabitants to allow them access to his realm, where their fears and insecurities would shape the realm around him, and allow their fears to feed and empower him.

And with so few mystically aware, odds were they would feel little save fatigue, at least at first.