The Signature Ability, or Sig for short, is unlocked at Persistent Level 21 via the Champion Tree. Much like Ultimate Abilities, Sigs are unique for each Immortal. However, Sigs are the most powerful abilities, even stronger than a fully leveled Ultimate Ability. It is first available/unlocked at Tactical Level 16.
Many new players confuse this ability with a passive/innate ability from League of Legends.