A trap with the blazing 666 glows on the area it is set upon. Enemies walk into it and unleash a minor demon to fight against. Higher your level the higher the demon in the traps becomes but it would obviously cost more the higher the level it is.
I'm not sure about this, but I don't dismiss the idea of a trap that summons some non keeper controlled entity to ambush would be attackers. I would rather have something like a strange rune or even a pentagram than the number 666 though.
I would prefer a summoning circle, really. Something along the lines of this:
(Taken from ~Zero-mostel's collection of circles at deviantart)
It adds to the atmosphere and whoever steps onto this tile will get his unpleasant surprise. Unless it's a succubus, that would only be surprisingly deadly.