Question

I met Landorus but did not catch him and he left. By accedent my son saved the game afterwards. How can i have Landarus to reappear to fight him.

Comments

All legendaries are do the same as UberPwnage said if you kill them

Answer 1

Defeat the Pokemon League (again or otherwise) and he will reappear at the Abundant Shrine. Again.

EDIT: As black empoleon said, this happens with all legendaries (3 Musketeers, Tao Trio, etc.) Defeat the Pokemon League again and they will appear in there respective locations again.

Answer 2

Defeat elite four!