1. Level Up:
* Most legendary Pokémon learn their signature moves through level up. This is the most common method. For example, Mewtwo learns Psystrike at level 68, and Rayquaza learns Outrage at level 68.
* Some learn it at a specific, relatively low level. For example, Giratina learns Shadow Force at level 30, and Dialga learns Roar of Time at level 40.
2. Through Evolution:
* Some legendary Pokémon learn their signature moves upon evolving. For example, Cresselia learns Lunar Dance upon evolving from a Cresselia.
* Some learn their signature moves upon Mega Evolving. For example, Mega Rayquaza learns Dragon Ascent upon Mega Evolving.
3. Special Methods:
* A few legendary Pokémon learn their signature moves through special methods. For example, Genesect learns Techno Blast when holding a Drive, and Hoopa learns Hyperspace Fury when it's in its Unbound form.
4. Game-Specific Mechanics:
* Some games have specific mechanics that influence when a legendary Pokémon learns its signature move. For example, in Pokémon Omega Ruby and Alpha Sapphire, Groudon and Kyogre learn their signature moves after being captured.
It's important to note:
* Signature moves are unique to specific legendary Pokémon, and they are not necessarily the strongest move the Pokémon can learn.
* The level at which a legendary Pokémon learns its signature move can vary depending on the game generation.
* Not all legendary Pokémon have signature moves.
If you're wondering when a specific legendary Pokémon learns its signature move, you can check the Pokémon's in-game Pokédex or an online database like Bulbapedia.