The Sleepers are the name of a series of five robots built by the Nazis during World War II. The first three Sleepers were highly-advanced for the time, and were designed to combine and share their powers. Captain America was able to stop the robots before they could be effectively used.
The fourth Sleeper (pictured above in custom Minimate form) was considered the most powerful of the Sleeper robots, with the ability to control its density and produce volcanic-level temperatures. The fourth Sleeper was activated years after World War II by the Red Skull and sent to destroy America. The combined forces of Captain America and Sharon Carter stopped the robot by causing a malfunction, leaving it intangible and powerless.
Some time later, Machinesmith was able to restore the fourth Sleeper, but with much less power than before. The restored Sleeper was then used by the Red Skull as part of the Skeleton Crew, his elite operative unit.