Sentinel is a campaign setup in the Mass Effect world in post Reapers era. The Synopsys is the following. The seat of supreme power in the known universe is now empty for the first time in billions of years. The reapers having built the Mass relays across multiple galaxies, the races of the milky way will come in contact with other factions and humanity can carve its place or trigger its extinction if the leader of that era takes the right decisions.
Multiple races across multiple galaxies will try to step into the void left by the reapers. Will humanity and the council races stand on the winning side of that arms race?