Exodia was, at first, a beast of such incalculable and intimidating might, that he was split up into five distinctive parts - each one of them being chained and then sealed away in a poem (hence "The Forbidden One"). This would prevent his power from ever being used again.

-- from The Book of Exodia, 5:36

封印されしエクゾディア (Exodia the Forbidden One)

功遜赤練蛇
容愧綠萼花
言惑嘉興粽
德慕峨嵋霞

封印されし者の右足 (Right Leg of the Forbidden One)

心吞造化腸猶斷
念起觀音目已聾
蝴蝶夢中譏馬白
寧為赤練不為龍

封印されし者の左足 (Left Leg of the Forbidden One)

耳順無聞清梵唄
目枯未結月明珠
掘田三尺公孫冢
但見髑髏顏色愉

封印されし者の右腕 (Right Arm of the Forbidden One)

南湖小荷纔聞月
北閣粗茶已醉人
穎脫無由何足道
酣眠袋裏夢游鱗

封印されし者の左腕 (Left Arm of the Forbidden One)

皓首一經孱
聚糧三世瘨
祗因不知味
夕死口頭禪

Were the five poems remembered by nobody, the five parts of Exodia would die with the poems. After that, Exodia Necross, the deceased Exodia, could be summoned via "Contract with Exodia".

-- from The Book of Exodia, 8:28

エクゾディアとの契約 (Contract with Exodia)

五五之年惟待化
黃粱好夢覺時癡
起身何急先續幻
半片明心終善思