Here's the note from the last chapter:
Utter this incantation!
If you don't, you won't be able to see mermaids.
I beg of you
Make my love come true
For me, [do something with] blood and flesh
(end of incantation)
(I know nothing about this manga. The last line of the incantation has an implied verb; one would need context in order to determine what verb should go there.)