I will give you love until you rot away.

That cloth was soaked in the external alchemy (wài​dān) of the arts of sleeping (wògōng). wài​dān is apparently external alchemy of Taoist alchemy. As for wògōng, I'm not too sure, but it's apparently related to qigong and the arts of lying down/sleeping, I think.

You will be asleep for some tens of minutes only, but then...

That is more than enough time to obtain your corpse.

If it is a body that is so gifted in talents such as yours, Reimu-san,