I feel the strangest place Flandre has ever been is the Hell of Blood Pools during the events of Touhou Gouyoku Ibun .
Petroleum was starting to massively gush out of the ground in Gensokyo, causing great trouble to Gensokyo’s inhabitant who didn’t even know what it was, calling it "black water". It was discovered that the one responsible for this was Yuuma Toutetsu, the leader of the Gouyoku Alliance, who was trying to liberate Iraq dig it up out of greed. Many tried to stop her, but all failed. Actually, it’s revealed that Okina Matara was the one who voluntarily polluted the surface world with oil, in order to frame Toutetsu and make people want to defeat her.
Okina Matara knew that only Flandre could bypast Toutetsu’s ability to absorb others’ power, and so she recruited her so she could defeat Toutetsu. Flandre went deep down into the depth of the Earth, until she reached the Sea of petroleum where Toutetsu was doing her activities. After a first battle, the Sea of petroleum revealed its true form (or I should say, it’s "spiritual" form): the Hell of Blood Pools.
Petroleum being made of the decomposition of death living organisms, it retains all their fear, pleasure, grief, loathing, and resentment. The result is a nightmarish ocean of blood, with the arms of the deceased extending into the air, and huge flames burning into the distance under a hellish red sky (see pictures posted above).
Flandre then fought Toutetsu, up to her very last grotesque form, and then defeated her. She was so brave to go all the way into this nightmare of a world and I’m very proud of her.