Abdel Adrian enters the battlefield, exiling Clone until Abdel Adrian leaves the battlefield, and creating a 1/1 Soldier creature token.
Clone leaves the battlefield, triggering its second ability, returning Fiend Hunter from exile to the battlefield.
Fiend Hunter enters the battlefield, triggering its first ability, exiling Abdel Adrian.
Abdel Adrian leaves the battlefield, returning Clone from exile to the battlefield as a copy of Fiend Hunter.
Clone enters the battlefield, triggering its first ability, exiling Fiend Hunter.
Fiend Hunter leaves the battlefield, triggering its second ability, returning Abdel Adrian from exile to the battlefield.
Repeat from step 2.
This elegant loop sets up an endless cycle of exile and return by turning Abdel Adrian, Gorion's Ward and a copy effect into mutual triggers. When you cast Abdel Adrian, he exiles Clone (which is waiting in the wings) and spits out a Soldier token, setting the engine in motion. By having Clone enter as a copy of Fiend Hunter rather than Abdel Adrian itself, you create a cascading chain reaction where Abdel Adrian and the Fiend Hunter-copy perpetually exile and re-enter each other, dragging any other nonland permanents you control along for the ride. The result is an unstoppable cascade of enter-the-battlefield triggers that floods the board with infinite creature tokens while keeping the rest of your team permanently safe in exile until you're ready to break the loop.