I've been thinking of ways to scare my guests this year, and I've come up with something I really want to try, but I'm not sure how to go about setting it up.

As is often the way, the party is likely to stay mostly in the kitchen, as it's a sort of "safe area" away from the scares and frights the rest of the house might hold. That is, until this year. We have a large patio door, and a fairly large garden.

My plan is to have someone scale the fence from next door (We're friends with the neighbours, and they'll probably be at the party anyway), and "appear" in the garden dripping with blood, carrying an axe etc. and then try to get in the back door.

Now, the dressing up shouldn't be a problem, and knocking together a quick fake axe wouldn't be a problem, my issue is that noone would be able to see the guy in the garden, and they wouldn't even notice he'd turned up.

Which bring us to my question, haha. I was thinking of setting up a flash of light/lightning to draw attention to him.
It would be the first time I've worked with custom electronics, if that is indeed what I would need, and I would like some tips or information if anyone has it.
my initial idea was possibly just a simple extension lead down the garden and a slow strobe light plugged in at the end of the garden, but I'm not sure if it would cope with being in the garden all night if it rained.
I'm on a budget, but if it's a cool enough effect I'll be convinced to pay for this!

Apologies for my first post being a "PLEASE HELP".