I want to make a Grim Reaper for a 40th birthday cake this week. (this is a small cake 8-9in round.... so he doesnt need to be big.) I want something more cartoony... less evil ... but if you have any insight on how to make one please let me know!
I'd go looking online for clip art type images of the grim reaper. Find something you're happy with, size it, print it, cut it out and then use it as a template.
Then you could go a couple different directions. Here are two of many possibilities...
Roll out some black fondant, about 1/8 - 1/4 inch thick, put your template on top and cut around it with an exacto knife.
Then you can add some details to it in greys or whites using either BC or royal (I'd go w/ royal personally). You can use a combination of piping and a paint brush to apply the royal.
Alternatively, you could find an image that would lend itself to doing it with a star tip (or black piping gel might look cool) on the cake. You just flip the image first on your computer, print, put template under wax paper and trace over the image with piping gel. Then lay that wax paper, piping gel side down, onto your cake and gently rub over the lines and it will give you your image outline to fill in.