I can't take the credit for this monster. It's based on a really old creepypasta.
It's like a silhouette of a cat, huge and with unblinking eyes. It will watch you when you arrive home at night- listening closely to the sound of the tumblers in your lock. Trying to scare it away won't work, as it will just return. Nobody else will ever be around to see it; the Intruder is your own personal stalker. Eventually, one night you will hear a key banging around inside your door- that means the key it has made is almost perfected, and it will be able to enter the very next night.
When it arrives, you must turn out all the lights in your house. Only a single candle can be lit- anything more will blind you from how bright all sources of light become once it comes inside. The light of the candle is your only real chance against the monster. Also, others may be present to finally see the being that is haunting you, but they are at risk as the Intruder will kill everyone inside the house on that night when it finally gets inside.
Intruder (6 HD, +8 To-Hit OR +8 AC, 2d8 bite, 1d6 claw, supernatural hearing, light blinding, predator hiss and feline agility)
The Intruder appears as a dark black cat of variable size. It chooses to stalk and kill one person, though will kill anyone else in the house on the night it makes its attacks. You cannot possibly attack it from stealth as its hearing is supernaturally powerful (it can hear a heartbeat). Once its key fits in your lock and it comes inside, the lights in the house become incredibly bright- a hearth fire becomes so bright as though it was the sun in your room. Electric lights are even worse. Using the light of a torch or Light spell would just blind you- fight as though you are blinded. Anything more then a single candle will be too blinding, though this light has no negative effect on the Intruder.
The Intruder is incredibly strong and agile. Its body is almost impossible to be damaged- except for right before it attacks. Every round the Intruder will alternate between preparing to strike, and then striking and retreating. Right before it attacks- it lets out an unnerving hiss and gains +8 To-Hit. It then attacks with its bite attack, and the next round gains +8 AC and will retreat while using only its claw attack. This switching pattern is the one saving grace of trying to fight an intruder.