If you cannot afford Phage on your first back, buy a Long Sword and Boots.
Build Ninja Tabi or Mercury's Treads instead of Boots of Swiftness if you're behind.
Build Spirit Visage instead of Dead Man's Plate if facing a team with no AD.
Build Spirit Visage instead of Death's Dance if an AP champ is fed.
Build Guardian Angel instead of Mortal Reminder or Lord Dominik's Regards vs teams with mostly DPS or burst.
Build Mortal Reminder over Lord Dominik's Regards unless the enemy team has no healing or lifesteal.
Build Oracle Lens over Stealth Ward if in need of objective control or facing a team with stealth.
Build Farsight Alteration over Stealth Ward if you did not build Oracle Lens.

Basic combo:

Q something for movement speed, activate Youmuu's Ghostblade, E W Q R, run away with stormraider's surge.

Maximum damage:

ignite auto W auto E auto Q R

Use the basic combo when fed or tower diving and the maximum damage combo for a lvl 6 all-in.

Flash is used instead of Ghost because this build already has a lot of movement speed.
Ignite is used to start snowballing.

Green - easy matchup
These champions are either forced to get close to darius and lose 1v1, cannot escape darius and lose 1v1, or are completely countered by Ignite.

Yellow - skill matchup
These champions are either ranged and difficult to Apprehend, beat darius 1v1 during their windows of opportunity, or are Darius.

Red - hard matchup
These champions outrange darius, don't rely on mana to deal damage, and are virtually impossible to Apprehend without a gank.