They see you
"Where our worlds follow a set of strict laws that govern all of reality, the Umbral Planes consider them nothing more than guidelines. This means reality itself is twisted within the Umbral Planes, bent and even broken according to the wills of those strong enough to dominate it. Concepts such as time and space have no place there."
Hao Xuan then finally pointed at the line separating the two groups of stones.
"This is the veil- an incorporeal curtain of barriers that keep the reality-bending energies of the Umbral Planes from spilling over to the material planes."
By now the flames were high enough to light up everything in the vicinity. The rocks closer to the firepit, the ones that represented them and the Material Planes were casting shadows that reached across the veil.
"Do you see those shadows projecting on the Umbreal Planes? That's exactly how we are."