The idea of the build is to buy seraph's embrace and liandrys to spam q on your enemies and keep them burning. Closer to the end of the game q will cd 3 seconds, w cd 2.5 seconds, e cd 7 seconds and r will cd 40-50 seconds allowing you to spam skills into teamfight and deal insane amount of damage as well as providing decent control. Your oneshot capabilities are very limited to enemy adc and maybe supports (not guaranteed) so the strength of this build really shows in extended fights when you can pour damage non stop.
Building void staff is not obligatory but it will provide extra damage, especially if the enemy have tanks or mr stackers with their filthy mercs.