The redevelopment of 850 Third Avenue, a Midcentury tower between East 51st and 52nd streets, has resulted in a $80 million investment by Waterman Interests and HPS Investment Partners. The owners of Kellari Hospitality Group, which owns Kellari Taverna, will launch a ground-level, 4,500-square-foot Greek and Mediterranean eatery called Artemis. The deal adds a touch of public glamour to the 21-story tower, which typifies the ongoing revitalization of the building. The building's comprehensive redevelopment includes all-new lobby, elevators, HVAC and other mechanical systems.