Have to praise the Dark Souls connection with the coffin-transport which is something I love to see. Might be the first time I've ever heard of an Annex Dungeon? I think this would be totally unexpected and if signposted to players should really make this a memorable sort of outing. Maybe they ignore it the first time and it pops up again in another dungeon? Lots of ideas.
I'm a little unclear on the total loot value here which seems low and there's a reference to some of it disappearing if carried back through the coffin which might feel unnecessarily punitive.
While some of the content is straight forward the framing of this odd extradimensional dungeon growing out of another one is pure gas and I'd like to see it further developed!