This build is for playing in a bottom lane, preferably with someone ranged, although nearly every champion is compatible with a Darius built in this way.
This build is focused on being tanky and being able to pick up kills because of that. You will have enough damage due to the Sword of Occult, and because of all the defensive stats it will be relatively easy to sustain in a fight. In this build end game I always end up having over 500 AD around 240 armor and 150 magic resistance (depending on the last item that I buy).

The last Blood Thirster is optional and can be replaced with Guardian Angel instead, it really depends on how good your team is and if you are the only tanky person on the team and if there is any AD carry. The rest of the items make Darius to be very balanced overall and deal immense damage picking up doubles and triples all the time even though he is nearly always being focused first. This build has very high cost efficiency.