To poison the Mongols’ fermented milk without raising the alarm in From the Darkness in the Ghost of Tsushima go through the tall grass on the left of the area, climb up and assassinate the guard. Keep going ahead and climb over the railing and drop back down on the other side.
Assassinate the guard ahead and keep going around the courtyard on the left side. Hide behind the wood until the guard ahead leaves then go up to and assassinate the guard. Go forward behind the fence wait for the guard to be standing in front of the fermented milk and cut across behind the guard.
Wait for the guard to leave and poison the fermented milk. Keep going around to the other milk sack and poison it too.