TL;DR

This variant of Scourge Arrow (SA) stacks APS (attack speed) and uses multiple initial arrows to quickly inflict multiple poison stacks on targets instead of relying on the poison spread from dead monsters. Unlike thorn arrows which deal 50% less damage and dissipate over a long distance, the initial arrows of SA deal full damage and don't dissipate, hence are much better for clearing. Scourge Arrow has 11.3 APS, 17.3mil sirus DPS at ideal condition, and fires 5 initial arrows with Dying Sun.

On the defensive end, we stack multiple layers of less damage taken such as 1.25x-1.4x Fortify, artic armour, Immortal call (50% uptime), etc., phys damage taken as fire/lightning, +#% to max fire/lightning resist, Wind Dancer, etc. Being a ranger, we can have high dodge chance and evasion. The build can tank all Sirus attacks (except the meteor maze blast), Shaper's tele-slam, Baran's dead bolts, etc.

For a more thorough breakdown, I highly recommend taking a look at Detailed build explanation (page 94).

Most of the gears in the build are self-crafted with Harvest mods. Harvest allows us to craft insanely strong items that increase the build's efficiency to an unprecedented level. If you haven't played Scourge Arrow before, want to explore a new playstyle, and enjoy crafting your own gears, you may consider giving the build a try. On a different note, it is reasonable to assume that we will not be able to retain such kind of power without Harvest deterministic crafts. Btw, crafting guide is available on the build's front page.

Example of the end-game helmet being used in the build. The crafting process is deterministic.

PoB for those who want a quick look. Highly recommend to open with the latest version of the community fork PoB.