Double Form is a Formchange in Kingdom Hearts III, triggered by the Oathkeeper and Oblivion Keyblades. It transforms Sora's clothes into a fusion of Light Form and Dark Form, and allows him to dual-wield Oathkeeper and Oblivion and harness the powers of both light and darkness.
Oathkeeper and Oblivion's Formchanges are nominally different: Double Form OKP (ダブルフォームY, Daburu Fōmu Y?, lit. "Double Form Y") for Oathkeeper, and Double Form OBV (ダブルフォームS, Daburu Fōmu S?, lit. "Double Form S") for Oblivion.
Double Form resembles a fusion of Light Form and Dark Form, with Light Form on Sora's right side and Dark Form on his left. He wields Oathkeeper with his right hand and Oblivion with his left; however, in situations in which he has to use one hand, he will wield the Keyblade that triggered the Formchange in his right hand.
A heart symbol is visible on Sora's sleeves and hood; on the hood, the heart is half in black on Sora's left side, and half outlined in silver on his right side. While the coat largely resembles Light Form, the left sleeve is black. The pants are black on the left and grey on the right, with grey back pockets; white kanji are imprinted on the front of each pant leg, with the kanji for "darkness" (闇) on the left side and the kanji for "light" (光) on the right side. The shoes are a fusion of Light Form and Dark Form's, with white collars, black toe boxes and heels outlined in white, and grey vamps and quarters. Double Form alters Sora's appearance in other worlds:
In Kingdom Hearts III, Double Form OKP is the second Formchange of the Oathkeeper Keyblade following Light Form, while Double Form OBV is the second Formchange of the Oblivion Keyblade following Dark Form. Double Form's fighting style is very similar to Roxas's fighting style.
The Finish command has Sora raise Oathkeeper and Oblivion skyward before changing the background to a sky at dawn. He generates a giant sigil beneath him before generating pillars of light to ravage the enemy.
Double Form is an ideal late-game Formchange. Though it doesn't quite match up to the immense damage output of Ultimate Form, Double Form is a viable alternative due to allowing Sora to stay in Form for longer, with Double Form acting as a secondary Formchange after the duration of Light or Dark Form. Combined with high single target damage output, high mobility, and solid defensive options, Double Form is optimal to use for many tough boss fights.
Some caution should be used when in Double Form, as the final combo finisher Mirage Thrust's long animation can cause Sora to become vulnerable in the event of an enemy counterattack. It's advised to keep this in mind for enemies that will counter at the end of a combo.