Your horrible trauma is felt by everyone.Whenever a Generator reaches 90 % of Repair progression, Merciless Storm triggers its effect:

• All Survivors repairing that Generator are faced with a continuous stream of Skill Checks until the Generator is completed.
• If any contributing Survivor fails a Skill Check or interrupts the action by any means, The Entity blocks the Generator for 16/18/20 seconds.

Merciless Storm can only trigger once per Generator per Trial.