Need ideas for subtle clues for a manor house

Hey all,

I have a section of my campaign coming up where the party will be investigatign a manor house. They have been told the ghost-assassin they are chasing is underneath the manor, and I need to lay clues for them to follow to work out the truth.

The manor is owned by an elf who is actually a warlock, who is also the assassin. Their patron is a powerful aboleth like creature in the underdark, who has a whole underdark town under it's thumb, controlling the guards and cultists, whilst other free-minded people live there, relatively undeterred by the whole arrangement (the Aboleth is slightly benevolent, focussed entirely on the acquisition of knowledge, and has become a Nothic). The underdark had symbols of an eye, which is the cult's symbol, and symbol of her patron. She has a secret tunnel in the house which leads to the underdark, and a room which contains her weapons etc.

The owner of the manor is a woman who takes in wayward souls and gives them a scrap of her power before sending them into the wide world, to escape their mediocre lives, and has become something of a patron. She is a high standing member of local society and widely regarded as a kind and generous soul, dedicating her life to rescuing the downtrodden (her methods of sending them to the world are not common knowledge).

In my world. Elves have the Light, which is a crystal which grants it's owner immortality, passed from parent to child. Some elves are impatient, or are third children, and they kill their family to secure the light. The warlock in the manor seeks these people out, forces a confession from them, then kills them. She then either takes their light, if they have no further kin, or she leaves it for their next of kin to have. The lights she takes, she gifts (through dark ritual which allows it to pass from one bloodline to another) to the wayward souls she has rescued, so that they can have immortality. She then sends the out to the world, so nobody finds out what she has been doing.

She has attacked one of the party and forced from her a confession that she killed her father for her light (in her backstory) so now has her as a target for her next assassination.

The party has a list of recent murders they are following and have worked out that the assassin is a vengeful ghost who kills people who have killed for the light (local rumours say she is a spirit, but she isn't). I need a way to tie up the Warlock in the Manor to the Assassin, so the party can deduce things in their own time.

Please help me come up with some clues! I have foreshadowed already that warlocks tend to change appearance, and the Warlock has met the party (as an upstanding person of society and as an assassin), so knows who they are and can suspect why they are there. The manor is a home for wayward folk who have nowhere to go, and is kept private for their safety (and to avoid people seeing the rituals).

Thanks!

One victim has the patron's symbol carved into their chest/ abolithic slime coating their wounds/ alien teeth embedded in them/ other ways that link the abolith to the killing that then links the warlock.