The rubber duck on the pool/bathtub?
I was going to say that there is so many unexplained things that its almost like finding a hay in a needlestack, and then you remind me about even more random things
The two most absurd things being Death trying to stop Soma and boss killing another one (Balore killing Giant Bat), for a bonus shot I'll use a hint to something that Im thinking for a time: Ashes, For ashes you need to burnm one of Dracula properties/powers is fire. Flame Demon is from hell and can be connected to Lucifer/Satan. From where I got this? Well, Satan's face is all over Aria's castle when you teleport.
I'll dismiss the "Satan's Ring" item because I know it was meant to be translated as Dark Lord's Ring. I really doesn't wanted to think it was Satan/Lucifer/Pazuzu because that reminds me of LoS, yet he hasn't appeared in the original timeline yet. Also he gets stronger with darkness in people's hearts and was said to appear in 1999 (even if they said that the prophecy was talking about Dracula, it could be talking about both)
I could go on and on, so its better if I stop here since I would consider that cheating lol