where is it? I must deposit the coffin but what I tried until now does not work

Inside the Secular Tomb in the NW corner of the Glatzburg Cemetery, move towards the room on the center right of the map which has two connecting passageways to the northwest and southwest. There is a green coloured sarcophagus with three lit candles and a skull in front which you can interact with. You will need both the coffin and holy water in your inventory to perform the interaction successfully.

Thanks

I would heave never figured it out