You have to give it the plates you find. He changes to the type that it would normally boost the power of.
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You will need to have Arceus to hold a Flame Plate and its Multitype ability will change its type to Fire. You can obtain the Flame Plate in Stark Mountain in Pokemon Diamond, Pearl, and Platinum. Alternatively, you acquire a Flame Plate by showing a hiker in Oreburgh Mine an Arceus in Pokemon Platinum only.
arceus is immortal.in jewel of life when dialga,palkia and girantina attack it change its form so that make arceus immortal
Arceus is a Normal-type Pokémon but if you give it plates to hold it will change its type to the plate's type.
to change it's type. (splash plate=water type,etc)
no arceus can not evolv but u can make him change type and form if u give hip a plate to hold e.g. flame plate turns him into a fire type.