RomRom is the great secret of Byrgenwerth. The Lunarium key tells us that “In the end, it is said, [Master Willem] left his secret with the lake.” Willem's wizened and mute form still bears life, of a kind, but can only point us towards the lake – and if you choose to jump in, you will find Rom.
It is worth noting that Willem as seen in the game has some sort of growths on his neck, similar to those seen on the Kin in the Orphanage, which suggest he either took his experiments too far or someone got to him. Given he is guarded by what looks like a Choir-affiliated hunter, perhaps the latter.
The Caryll rune Stunning Deep Sea also implies that Rom's lake was a guardian of some kind, keeping either the Great Ones or their would-be servants numb to the truth. But we kill him before realising any of this. Oops!