Blackthorn
Rek'Sai holds the items and Malphite holds the line. Everything else is bought to be sold later — the early Blackthorn units are storage, not the plan.
Spot it
You see it when Blackthorn units stack up early, and above all when Kha'Zix drops from an orb.
The board

Rek'SaiItem holder
Carries the items that end up on Malphite.

WarwickItem holder
A holder, nothing more — build items that will be good on Azir, Ahri or Kha'Zix later.

VeigarItem holder
Same job as Warwick. Do not build for him, build for who comes after.

AzirFlex
Blackthorn, Executioner and Summoner at once — three-cost, rerollable at level 7.

Kha'ZixLate carry
Not a Blackthorn unit at all (he is Rival) but the reason this line wins: he evolves Executioner and Ravager on himself.
Items
Gargoyle Stoneplate
Malphite is the strongest solo tank in the set, so Gargoyle Stoneplate comes first. Component leftovers go toward the late carries, not toward the holders.
Augments
"Solo Plate" is the pick on 2-1. "U.R.F" or "Urf's Gambit" — or a carousel spatula — gets you the Blackthorn emblem (Spatula + Belt) that turns Kha'Zix into the sixth Blackthorn.
The decision
Emblem or no emblem. With one, six Blackthorn buffs the whole board and the hypercarries take over. Without one, you play the tempo line instead.
- Azir / Kha'Zix reroll — Emblem plus an early Kha'Zix
- Blackthorn Ahri — Emblem, but rushing level 8 feels better than rerolling
- Zyra Soraka AP — No emblem, but a strong early board
