I won once dressed as "the Dark". Costume was simply black sweats, black gloves, black face paint, black hair color. The overall effect (unintended) was rather like a (rotund) version of Nightcrawler from X-Men.

But the critical component was a homemade upper palate made of Friendly Plastic, with a "seed" light bulb embedded in the material. Wires ran out the corners of my mouth to a battery pack taped behind my neck. When I talked, the light would spill out of my mouth. The effect is really startling. Accidentally scared the absolute crap out of the little kids at the party (and then in trying to reassure them, the natural thing to try is a broad smile, which made things worse...;-), but even the adults would shudder.

Making the denture plate was a bit of a trick. You need to start with a flat plate of Friendly Plastic of the right size to fit your upper palate right behind your teeth, get it hot enough to be pliable (in hot water), then stick it in your mouth and push it up against your upper palate with your tongue, without folding it or stretching it too much in the process. Be prepared for several tries. Luckily, the
stuff is reusable, if you are willing to heat it and roll it out flat again.) Once you have a good fit, add the light source and cover the leads with another layer of Friendly Plastic. (don't cover the light source, the plastic blocks too much light.)

You can buy commercial LED "denture plates", but the effect is usually not so good, because they sparkle in a clearly electronic way. The motion created by talking produces a nice organic flicker which seems to work better by itself. Plain old white light worked really well, but red or orange LED would probably be a good bet these days; more light for less heat and less battery drain. Maybe a blue one and an orange one together, to look like fire?