Builds: Shadowfiend

Shadowfiend (early effectiveness)

Start (625): Circlet, Slippers, RoP, Healing Salve

Early: Boots, Ring of Aquila

Core (go gank): Phase Boots, Shadowblade, TP

Core Ext: Crystalis,  BKB

Luxury (6-slotted): MKB, Butterfly, Deadalus, Satanic, BoT, BKB

The following build provides "lean" item progressions for Shadowfiend, where he will start coming online as a ganker quite early, with minimal farm:

Shadowfiend Midas Build:

Another build is the "Midas" build, where you will be aiming for slightly later impact, but stronger late-game presence with higher farm:

Start (575):  Tango, 3 Branches, Circlet, Healing Salve

Early:  Boots, Gloves of Speed

Core:  Midas, Power Treads, Shadowblade

Core Ext:  BKB, HoD

+ Attack modifier item (choose 1):

Diffusal, Desolator, Satanic, Skadi

Luxury:  Manta, Butterfly, BoTs, MKB

Shadowfiend Split-Pusher Build:

This would be a very-situational build, where you'd want Shadowfiend to be able to push a lane by himself (which he can do well with flash-farming), but also be able to escape at a moment's notice, or switch lanes easily:

Start (625): Circlet, Slippers, RoP, Healing Salve

Early:  Boots, Aquila, Maelstrom

Core:  Shadowblade, BoTs, Blink Dagger

Luxury:  Mjolnir, Manta, Skadi