The Failed Summoning

At the time of The Founding, five Great Vampire Houses shared their power over the City, as decreed through The Last Alliance. But as is common history in vampire politics, one of these Houses grew overly ambitious, and this led to their demise.
This was the House of <Insert Cool Name Here>. Legend says they planned to overthrow the other houses by attempting to summon a terrible, ancient power from another plane. The Summoning Ritual went horribly wrong, and the destruction that followed took the entire house down with it. The details of this event are all speculation.
What is for sure is that, today, the ruins of their mansion stand still, untouched, likely warded by some Magick, so to serve as a reminder. This is a cautionary tale for vampires and humans alike. The Order is settled, so They say.