Careful Planning (Part 1)
"At that point, all I had to do was to fill the Verhen Mansion with Tiamat Fear with the help of my Demons and make the intruders waste enough time so that their cores, bodies, and even their weapons were infected by world energy carrying our will."
While Dragon Fear affected solely the magical aura surrounding a Wyrm, infusing it with their killing intent and making it almost tangible, Tiamat Fear would also spread to the surrounding world energy thanks to the power of Lith's eyes.
A power that now Tista shared and so did his loyal Demons. They had all received through him a boost to their abilities from the tower and used it to spread the Tiamat Fear while pretending to patrol the corridors.
At that point, with each second the undead had spent inside the Mansion, with each spell they had cast, the infected world energy had replaced that stored inside their bodies, building up slowly until it had become dominant.