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Okay so here's all the info about Dialga and Palkia's new forms
>キングのすがた (Appearance of a /King/), localized as "Lord Form".
Basically an extension of the "Lord Pokémon" you fight and calm in the story, ie: The special バサギリ Basagiri (localized as "Kleavor") is called 荒ぶる森キング バサギリ Wild Forest King Basagiri (localized as "Lord of the Woods Kleavor"). Dialga and Palkia simply get King/Lord forms that change their actual look rather than just make them glow.
>100/100/120/150/120/90
>90/100/100/150/120/120
Both legendaries trade 20 points of Attack for a Special Defense (for Dialga) or Speed (for Palkia) boost, so the BTS is unchanged, as with Giratina. Ability is however the same as before, Pressure or (if hidden ability) Telepathy, but these are only coded for future games, they can't benefit from them in Pokémon Legends Arceus (same as happened with Meltan and Melmetal in Pokémon Let's Go! Pikachu/Eevee).
>だいこんごうだまGreat Diamond Orb and だいしらたま Great Pearl Orb (localized as Adamant Crystal and Lustrous Globe
These new items replace the old こんごうだま Diamond Orb and しらたま Pearl Orb (localized as Adamant Orb and Lustrous Orb). When used, they change the form of their respective legendary into their new forms.
>光明 フォルムチェンジの 引き金。 創造神に 酷似する姿こそ 真なりと 思わざるを得ぬほどの 剛力 発揮す。(Form change caused by exuberant light. Due to its power and resemblance to the creator god, one can only think it's the real deal.)
>天翔ける姿 万物を 創りし 存在を 想起す。 姿を 模すこと すなわち その力を 我がものとする術と 推察。(A figure that flies in the sky and reminds us of the being that created all things. I presume it imitates its form in order to gain its power.)
Localized as "Radiant light caused Dialga to take on a form bearing a striking resemblance to the creator Pokémon. Dialga now wields such colossal strength that one must conclude this is its true form." and "It soars across the sky in a form that greatly resembles the creator of all things. Perhaps this imitation of appearance is Palkia’s strategy for gaining Arceus’s powers."