If I attack a battlefield that Ahri controls and my units get -1, then I use RTW (Rift the World) to bring another unit onto that battlefield to join the showdown, does Ahri's Legend ability re-trigger?

Ruling: Yes, Ahri's Legend ability re-triggers. She triggers when a unit gains the attacker designation at a battlefield her owner controls for the first time each combat, regardless of the timing.

Nuances:

• Attack and defend triggers are not limited to the start of combat
• Each unit that becomes an attacker triggers Ahri separately when they first gain that designation during a combat