This theory is as follows: Ieyasu was killed by Nobushige SANADA in Osaka Natsu no Jin (Summer Siege of Osaka), and a shadowy person acted in place of Ieyasu until he died due to illness, to avoid the confusion that would ensue otherwise and to make the work to stabilize the bakufu progressed smoothly.
大坂夏の陣の際に家康は真田信繁に討ち取られ、混乱を避け幕府の安定作業を円滑に進めるために影武者が病死するまで家康の座を代行させていたとされる説。