A mirror, should the eldest living creature in the room look into it, shatters violently for 1d6 piercing and slashing damage, and a 50% chance of blindness for 1 hour.
Optionally, to add insult to injury, if you own the Tome of Beasts from Kobold Press, see the Mirror Hag on page 243 for an accompanying, thematically consistent foe.
Otherwise, any wight or similar incorporeal entity may be summoned by the obligatory bad luck accompanying a broken mirror.
Broken glass by Max Pixel. Creative Commons cc0.