What signs for Illuminati?

The 'sign' used by the Illuminati was the 'Owl of Minerva' from Greek mythology; an owl perched on an open book. A symbol representing the scholars who formed the organization.

The Illuminati was an organization (1776-1785) of enlightened intellectuals whose goal was the overthrow of an oppressive monarchy and the control of civil authorities by the Roman Catholic Church. They were very unsuccessful in their endeavor because that monarch successfully destroyed the organization and imprisoned their leadership.