The Helm of Awe, or Ægishjálmr, was a magical symbol worn by Norse and Icelandic warriors for protection in battle and to invoke fear in one's enemies. How precisely it was to be "worn" varies a bit with time and place -- sometimes it's drawn on the forehead, sometimes tattooed on the body, and sometimes it's meant to represent an actual helmet (as in Wagner's Tarnhelm). The web is full of modern-day drawings and tattoos, but I wanted to show (at right) a scan of a page from an actual Icelandic grimoire that highlights the original symbol.
There are a lot of interesting things about Icelandic symbol magic that I could talk about, but you can get a feel for a lot of it from the above links. One thing that particularly jumped off the screen at me, when I first saw the Helm of Awe, is its eerie similarity to magical symbols drawn centuries later, half a world away... Have a look at a Haitian Voudon symbol (Veve) for the imposing Baron Samedi:
In RPGs, there have been some products that describe a kind of "Tattoo Magic." See a Pathfinder source book, as well as a page of Rolemaster house rules. Borderline creepy that they are, they still don't seem to approach the religious aspect of both the Helm and the Veve. Perhaps it's the CLERICS that should get these kinds of powers when they choose to mark themselves with the symbols of their gods. Of course, if it's just drawn on (and not permanently tattooed) then it wouldn't impart as strong a bonus. And woe to the cleric for whom the drawn-on symbol accidentally rubs off!