The Sacrifice Engineer was an Engineer chosen by the elders of his race to serve as a sacrifice in the process of seeding a planet.
When the Engineers visited a planet, the Engineer was chosen by the elders of his race to serve as a sacrifice in order to seed the planet with life. The Engineer stood over a waterfall and after seeing a ship, the Engineer removed his cloak and ingested an unknown substance that was given to him by the elders. His entire body structure began to dissolve and he fell into the waterfall, seeding the planet with his DNA, and hastening evolution upon that world.
The scene of the Engineer being sacrificed to create life is a reference to the myth of Geshtu-E (also called Geshtu or Gestu) , a minor "God of Intelligence" from ancient Sumerian and Akkadian mythology. According to that legend he was sacrificed by the greater Gods so his blood could be used in the creation of mankind.