Spoiler warning: this article may contain major plot or ending details.
When the shrine is interacted with, you will be asked if you want to etch something on the shrine. If the phrase "THANK YOU..." (using the ellipsis character) is written, a clearing will appear ahead, where Shaymin can be battled and captured after a event "borrowed" from Mew's. Shaymin is level 30, has the ability Natural Cure, and knows the moves Growth, Magical Leaf, Leech Seed, & Synthesis.