How do I defeat the noonwraith in the Devil by the Well?

After some more researching, I found an interesting post on GameFAQs, namely:

I spent a while on this. I searched everything and even fought it once. I had to reread my bestiary to trigger the body burning interaction.

And sure enough, after reading the bestiary entry for the Devil by the Well (again), a cutscene was triggered and now I can burn the skeleton instead of just examining it.

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TL;DR

Read the bestiary after reading the diary, finding the bracelet, and pulling out the skeleton.