Wizards of the Coast has a new in-store program in which characters will face 'mega-encounters' that will test players' tactical skills and abilities to adapt. Each encounter should take only a few hours, but should be challenging enough that it may take several attempts. More details will be released over the next few months, beginning with Character Creation rules in June. The Lair Assault itself begins September 1st.

You can check out the Lair Assault site here.