Two stories from the Grimm brothers with people going beyond death. On the first, it's a prince and princess who find love...after they swear a terrible and toxic oath to one another. On the second, if your guest sees ghost children in your house, maybe you should listen to them?
The creature is the Leyak, a witch who likes munching on some human entrails and turning into a monkey with golden teeth.
Scoundrel: If someone offers you a miracle tonic that's literally just heavily irradiated bottled water, don't take it! https://myths.link/scoundrel