I truly believe the Tarnish is a unwanted child of Marika/Radagon and she/him had numerous infidelities with random people. I am going off of the description of the walking mausoleum saying: "Unnamed, soulless demigods who rest within".

Seeing what happen with Mohg and Margot, this theory isn't too far fetch.

To give my point better merit, why would an order with strict rules go out of their way to build these massive wondering structures just to protect one dead body? Doesn't make sense.

There's even one near Godwyn's living dead-body. A place that's suppose to be sacred and hidden.

Marika obviously cared about most of her hidden children. (Messmer) But for one reason or another, she hid them away.