What is "cardhalla"

Cardhalla is a charity event that has been held at Gen Con since 1999. During the convention, a city is constructed out of donated trading card game cards. Near the end of the convention attendees are invited to throw coins at the city to destroy it. The thrown coins are collected and donated to charity. The motto is: "Build. Donate. Destroy."

The name is a portmanteau of "card" and the Norse city of Valhalla.