How did Robbie's demon come to life?

From the text:

A throng haint stepped up, momentarily engulfing Wick, and then disappeared. Robbie’s demon writhed to life, snarled, and hurtled out of the wall. It became three-dimensional, a towering monster glistening with fresh paint, long arms stretched out in front of it as it raced back and forth. “You know what to do,” Wick said. The demon haint bellowed and then rushed down the stairs and out of sight. “They’re going for our people,” Robbie whispered.