Wick
There are flowers growing around a heart with a wick sticking out of it. The heart is a sacrifice to the old gods and the life energy bleeds from the burning wick to feed the flowers. The wick burns, releasing the power of the heart which spreads into the ground around the heart causing the flowers to grow.
See this morbid scene. As the wick burns you can smell the smell of burning organs. The flowers are pretty though.
しん
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Sometimes the wick burning the heart isn’t enough for the flowers to grow properly and they get thirsty for the blood from your shin (しん). So, you slice open your shin with a sacrificial knife and drip your blood over the thirsty demon flowers.
Imagine slicing your shin with a knife, causing blood to drip on the flowers around the heart with a wick in it. Your shin really hurts from the injury, but the flowers are rejoicing as they’re showered in blood.