The Silver Surfer serves which Marvel character?

The Silver Surfer cruises through space on his cosmic surfboard looking like the universe's most stylish lost soul. This chrome-plated philosopher has the power to manipulate matter, travel faster than light, and contemplate existence while maintaining that perfect metallic glow. But all that cosmic power doesn't mean he's free. The Silver Surfer works for someone much bigger than himself. Which Marvel character does the Silver Surfer serve?