The following is a series of excerpts from the private journal of Lord _________, which was found by an adventurer in the lord’s abandoned (and some say haunted) manor.
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Entry 1
I have just gotten the most wonderful gift for my wife. ‘Tis a full length mirror in an ornate, gilt frame. Elisabet adores it. I am glad to know I made a good purchase.

Entry 2
The scholar Fellis visited my home today. It was a fine visit, until he saw the Elisabet’s mirror. He became quite agitated. He claims it’s a dangerous thing to have about and that it should be destroyed. He upset Elisabet a great deal. I have never seen Fellis behave in such a manner. It worries me.

Entry 3
I am becoming quite disturbed. Elisabet spends all of her time in front of that mirror, staring into it in fascination. I have never known her to be an extraordinarily vain woman, but the mirror seems to have made her so. She no longer wants to leave the manor or play with the children.

Entry 4
Lately the children have complained of nightmares. They say that their mother’s mirror is a monster, waiting to eat them up. I console them as best I can. Elisabet has ceased to even acknowledge me or the children or anyone else outside of that damnable mirror. I’m beginning to realize that Fellis may have been right.

Entry 5
The children are still having nightmares, and apparently they’re getting worse. I daren’t tell them that I’m starting to have them too. So are the servants, judging by the way they avoid Elisabet and the mirror. I worry for her health; she has stopped eating properly. One of the maids has vanished. I assume she was frightened away. I shall write to Fellis and ask his opinion on matters.

Entry 6
I have become irrationally terrified of that mirror, to the extent that I avoid it when I can. Fellis promised to do what he could about the mirror, but I’m afraid it may be too late to save Elisabet. She has not moved nor, to all appearances, slept in three days. Everyone in the manor has continued to have nightmares centered on it. The servants are talking about leaving.

Entry 7
God help us all. I’m certain now that there is a malevolent intelligence in the mirror, and it’s taken Elisabet. True to his word, Fellis arrived to do what he could concerning it. I had assumed that Elisabet would pay no heed to him, as she does to everyone else. I told him all that has occured, then left him to do his work. An hour later, I came to check on his progress. I found him dead in the hallway. There was a look of terror on his face. I found Elisabet the same way she had been when I left. But there was blood on her dress.

Entry 8
I dealt with the death of Fellis as well as I could; money does wonderous things. But it cannot give me back my friend, nor my wife as she was. Many of the servants have left. Those who remain refuse to go near my wife or it. I have written to my sister; I feel I must get the children away from this place. Away from her. Away from it .

Entry 9
The children are gone, my sister has taken them. All but three of the servants have fled, and those three will not stay long, I fear. Were it not for Elisabet, I would leave. But I still hope that she could be saved. Her health has declined, but not nearly as much as it seems it should have. The nightmares have continued to worsen, and I make no secret of the fact. I feel like I should look in the mirror, though I have not done so yet. I fear to.

Entry 10
All but Calun are gone now. Calun has been with me for years, and has sworn not to abandon me. Last night, he caught me sleepwalking towards the mirror room. He has also confessed that he feels driven to look at the mirror. I have taken the precaution of locking the doors. I no longer hold out any hope for Elisabet. She seems more like a porcelain doll than a woman now. I have begun making plans to leave.

Entry 11
I’m afraid that I shan’t be able to go after all. Every time I try to, I find myself stopping for no reason. I’ve stopped sleeping, for everytime I do, I have those horrid nightmares and feel that compulsion to go to the mirror room. What worries me, though, is that I cannot find Calun.
Addendum: It has Calun. I found that doors to the mirror room had been forced open. Calun was there, standing behind my wife, staring into it as raptly as she. It seems that the effect is much more rapid now.

Entry 12
god help I saw help so many faces key wheres the key musn’t get me too the lord father faces the Lord Father which art in heaven saw the Lord Father in heaven looked sawfaceshelpmirrortheLordFatherwhichartinheavenhallowedbethynamefaces ... Note:From here it becomes unintelligible scribble.

Addendum: I’ve calmed down now. When I realized where I was, I was locked in my study saying the Lord’s Prayer over and over again. I looked. God help me, I looked, a split second before I realized what I was doing. I saw Elisabet and Calun in the reflection, of course, but so many others as well. I saw the maid who had vanished, and Fellis, and many others that I didn’t recognize. I believe it was instinct that made me run and lock myself in here. Something must be done.

Entry 13
It almost had me again. If I look again, I’ll be lost. I have decided. I must break the mirror.

Entry 14
I have done it. Before I (or anything else) could stop myself, I smashed the mirror. I had given Elisabet and Calun up for lost. When I swung the heavy candleholder I had picked up, there was a scream as if from hell itself. Calun and Elisabet leapt at me, to try to stop me, but they were too slow. When I broke the glass, they fell as if dead. When my heart stops beating so fast, I shall go and check.

Entry 15
It’s… what have I done? I cannot write…
Addendum: I am lost. When I went back to the mirror room, I found a scene that horrified me. The mirror was intact, though I still bore the cuts I had recieved from falling glass. Where Calun and Elisabet had lain, however, was only a pile of clothing and shards of bloodstained glass. I am intuitively certain that I indirectly killed them. I am also certain that sooner or later, I’ll drop my guard and the mirror will have me. And I fear it may be sooner. I can hear whispers in the hall.

There are no further entries. The final entry is immediately followed by a large inkblot and a good deal of a brown substance that bears a disturbing resemblance of dried blood.
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Some ideas:

The lord got the children away from the mirror before it could get them, but that is not to say that the children were unaffected. So maybe the children have become evil, and end up having some Spooky Psychic Powers involving mirrors. So you might have a few evil adults doing Bad Stuff, and it leads back to the lord and ultimately the Mirror. Alternatively, this could be a much more recent event, but when someone goes looking for the children, they’re missing. Quite possibly doing Bad Stuff. Possible origins include being some necromantic/demonic focus or Experiment Gone Horribly Wrong. People the mirror takes? Possibly energy sources so people can do Bad Stuff, or maybe the mirror ‘feeding’ itself.

Another thought: the mirror protects itself. In the diary, it is strongly implied that it used the wife to kill the guy who was gonna mess with it. People who get too close or who know what it is should watch out.

Could it be that it is a family thing that the mirror is after or else the servants may have been more affected.
Perhaps it is after only those with noble blood as a primary objective while any others may be convenience kills or simply extra challenges that don’t really matter.
Fellis seemed to know something, what kind of house does he run and what would somebody find there if they went searching?

Ready Made Adventure: Have the characters get a letter requesting help. (the noble may be a relation, acquantence, friend of the family…) When they get there they find the scene as described and find the journal able to read the events while they were enroute.
When the characters go to find the children then they find that the sister has been murdered and the children gone like Ria mentioned. Could either be the noble went to reclaim them in his madness or the children were already turned by the mirror.

Magical Properties:

No one knows for sure.  But they are Spooky! so make note of this.