When this Final Smash is used, Chrom will dash forward while shouting "Anything can change!", "Now I'm mad!" or "Your end has come!" and strike foward with his sword, and any opponent that he hits with it will be stunned. If all of his opponents are hit, the camera angle changes to resemble that of attacks in Fire Emblem Awakening. Chrom then back flips away, then dashes forward while attacking the stunned opponents again with a more powerful strike, sending them flying.
Aether is a skill from the Fire Emblem series, originating in Path of Radiance as Ike's mastery skill. Chrom can learn the skill in Awakening as a Level 5 Great Lord. Lucina always begins with the skill as her inherited skill from Chrom, as well as Kjelle or Cynthia if Chrom is their father (regardless of whether or not he has it equipped as a skill), and Morgan if Lucina is her mother. In addition, certain versions of Chrom can learn it naturally in Fire Emblem Heroes, while others can learn it through skill inheritance.
All three of the lines Chrom can yell when activating this move are lines he says when performing a critical hit or activating an attacking skill (such as Aether) in Awakening. However, in Awakening, he said "Now I'm angry!" instead of "Now I'm mad!".