Hidden monster in a house quest

Quest

A cleric of Erathis named Melissa meets up with the PCs and asks
them to come along to her temple.
In the small temple lies a body, prepared for burial.

When she lifts the veil covering it, there are bite and claw marks
on the body of the human male.
She tells the PCs:
"This body was found 4 days ago in a dark alleyway.
His name is Kris, a baker nearby. We don't know who would want him
dead, so I prayed to the gods for an answer.
They show me visions of a house nearby, I know where it is.
I know the inhabitants, a nice couple:
a female Tiefling named Hope is in there with a human male named Brandis.
I didn't go to them yet because the monster might still be there and
attack on being provoked.
And I don't want to worry them by telling this, for now they seem safe.
I tried to get them out of the house but they wouldn't leave.

So could you go to the house and look if my visions about the monster are correct?

As a reward I'll give you a free Holy Water (level 11; see Divine Power rituals)"
The Holy Water is also for sale at 350 gp.

Examining the body

A Nature DC 15 reveals that the bite marks are quite small, so the monster may be a small creature or a creature with a small mouth.
A Heal DC 16 reveals some necrotic residue in the claw and bite marks.
A Religion DC 16 with the necrotic residue points to the monster being undead.

Going to the House

The neighborhood in which the house is located is filled with
Tieflings and halflings and they notice the PCs entering and leaving.
Hope answers the door, she doesn't want anyone entering the house but will
eventually let the PCs in.

A female Tiefling named Hope is in here with a human male named Brandis.
They have an adopted human child named junior.
The windows are barricaded shut, Brandis is busy in the kitchen chopping up meat and junior is crawling through the residence.
There is one large bed for the couple and a small bed for junior.
There are 2 closets containing used clothes.
There is only one light source in the building, the brander.

Hope is sitting at a table and invites the PCs to sit down.

There are some forks, knives and plates on the table with
the forks and knives becoming a bit rusted.
She has a high Bluff (Insight DC 21) and will say she knows
nothing about the murder or the monster.

When her Bluff fails her or Brandis comes back from the kitchen,
the following events happen:
Junior takes out a filthy razor from inside his clothes
and (surprise) attacks one of the PCs.
Brandis takes out a large kitchen knife and Hope kicks down the
brander, making the room pitch dark.
Hope grabs one of the rusted knives and attacks the PCs.

Combat Encounter

The parents are unrisen Adults, the child is a Corrupted Offspring.
They can be found in Open Grave under Unrisen.
Unrisen are people who came back wrong after a Raise Dead ritual.
They are undead but look normal, they try to fit in.
As undead they possess darkvision and have the advantage with the lights out.
There is only one light source in the building, the brander.
Their tactics is to let the child surprise attack one of the PCs and then put out the light, then use their darkvision to kill them.
Unrisen look normal in all ways. They can be detected with
Holy water (a few drops are enough) or the Corpse Light ritual.
When they are near defeat, they try to run outside and make the neighborhood side with them.
Should that happen Melissa will help the PCs when asked and use drops of Holy Water to prove the nature of the unrisen.

Level 10 encounter:

• Corrupted Offspring (vulnerability to radiant reduced to 5)
• 2 Unrisen Adults (same stats as Corrupted Offspring, except they have only this attack:
  Rusted Knife (At-Will), +16 vs. AC,    3d6 + 6 necrotic damage