Valina's Arceus Event!
I won't tell where but there is a statue of the Almighty Arceus somewhere in Valina. It has has a tablet on it that reads:
"__e __g__y __e s__n_s __r_ ___or_ y__. T__ _o__r__l b____ __at c__a___ ___ee __v__e __w__s: ___p__al ___, _p__i__ O__, a__ __e _e____de O__. _____"
"The text is too worn out to understand."
If you go to a certain place, and old man will tell you the story of Arceus. And afterwards, he'll give you the Missing Tablet, which you can bring to the statue and place on the old tablet. It will then read:
"The Almighty One stands here before you. The Powerful being that created three divine powers: Temporal One, Spatial One, and the Renegade One. アルセウス"
After this the screen will transition you to a cosmic world where Arceus will appear and you will battle it. After defeating or catching it, you will return to the statue. It will then read for then on:
"The Almighty One rests. The Powerful being that once created three divine powers sleeps. The Alpha One. アルセウス."