After an arduous battle with Eterwhatever, you rush to the closest Pokémon Center available (as it is your only option, because trying to go anywhere else ends with you blocked by invisible walls that say "I better go to the POKÉMON CENTER first."). As you try to talk to the nurse, a generic elderly male NPC approaches you for an unavoidable cutscene. This specific NPC, you vaguely remember, stays on this Pokémon Center and gives you a POTION when you talk to him for the first time.
"Hello there young trainer! Yer POKÉMON are looking much stronger than
the last time I saw them. Reminds of me old days, ha ha ha!
I was clearing me house these days and found an old book about
to the legends of the region. Ya see, according to it, back when our world was still...
Oh, look at me. Rambling about old tales to an uninterested young'un, ha ha ha!
I apologize. In any case, this old POKÉBALL was near the book when I found it.
I think something is inside it but I am not a TRAINER anymore. Would ya like to keep it?"
> Sure.
> I want it!
"Ha ha ha! Fantastic! I knew ya had it in ya, kid! Take care of whatever is in it!"
Would you like to rename ARCEUS?
ARCEUS was transferred to the PC Box
Confused, you talk to the NPC again on his usual corner of the Pokémon Center hoping for an explanation for what just happened.
"Hey there young trainer! Hope that POTION I gave ya was useful!
It's important to know when to use ITEMS in battle, ya know? Ha ha!"
No matter how many times you try, the NPC is back to his usual pointless dialogue. So you check the PC Box and sure enough, Arceus is in there.