The release of Diablo III is imminent, and with it, Blizzard has launched a campaign to promote awareness of the game called Darkness Falls, Heroes Rise. The campaign was launched a month ago, and it's coming to an end in ten days from now.
More than just an advertising slogan, the campaign offers anyone who purchases Diablo III with five in-game sigils as long as they participate it. These sigils are limited for a very short time and will be completely unavailable upon the game's launch, so now's a good time to grab all five of them.
Each of the sigils you unlock are based on each of the five player classes who represent Diablo III's heroes. Once unlocked, they'll be tied to your player account for use at any time in-game. You can obtain all five classes.
Be sure to participate in the Mark of Valor challenge here.