Ultramarine Orb Elixir
The Ultramarine Orb Elixir (紺珠の薬 kanju no kusuri) is a medicine created by Eirin Yagokoro to assist in solving the events in Legacy of Lunatic Kingdom. The elixir is apparently known to all Lunarians.
Believing a Lunarian invasion was underway in Gensokyo, Eirin created the medicine because, due to the strength of the Lunarians or circumstances involved, humans were too weak to stand against them as they were, she couldn't get personally involved, and the youkai were unable to see the problem since the lunarian probe made completely impure beings like them unable to see the invaders, as the lunarians didn't want to provoke and fight the youkai. Reisen Udongein Inaba was then instructed to deliver it to Reimu Hakurei, Marisa Kirisame and Sanae Kochiya to deal with the threat. This was the Ultramarine Orb Human strategy.
The medicine temporarily removes the impurity of death and allows the user to experience the immediate future. In the game's Pointdevice Mode, this meant players could restart from the end of the previous "chapter" upon death, rather than at the beginning of each stage. Players have the option of not taking the medicine by choosing Legacy of Lunatic Kingdom's classic mode, Legacy Mode.
The Ultramarine Orb (紺珠 kanju) portion of the name is also part of Legacy of Lunatic Kingdom's title. It may be a reference to Zhang Yue, a prime minister of China's Tang dynasty. He owned a certain gem which, if he ever forgot something, he could hold it in his hand and remember.
The Ultramarine Orb Elixir was canonically not used by the protagonists during the events in Legacy of Lunatic Kingdom. In an interview, ZUN revealed that the canon ending is a no-miss clear in Legacy mode (the heroine elects to not use the Ultramarine Orb Elixir, but still manages to not get hit).