Salvation Lies Above is an achievement/trophy for the Zombies map Tag Der Toten in Call of Duty: Black Ops 4. It is the main story Easter Egg of Tag Der Toten, and requires players to "ascend from darkness".
The name of the achievement is most likely a reference to the message on the wall of Nacht der Untoten stating "Salvation Lies Above".
Furthermore, the description for the achievement is most likely also a reference to another message on the wall in Nacht stating "Ascend From Darkness".