The Lumenwood Gardens is an area located in in the Old Hunters DLC. This area is accessed via the Research Hall.
Lumenwood Gardens acts as the arena for the second, non-optional boss of the DLC - the Living Failures.
After defeating the Living Failures, players will be able to travel to the Lumenwood Gardens boss lamp via the Hunter's Nightmare headstone in the Hunter's Dream. Lady Maria of the Astral Clocktower, the DLC's penultimate boss, is located in the Astral Clocktower directly adjacent to the Lumenwood Gardens.