User blog comment:Xilinoc/Ch. 574 DEATH IN VISION/@comment-24739129-20140326185729

Agreed. Looks like Gremmy's power is more akin to "Suggestion". It says so in his name after all..the "Visionary". Another term for Visionary is a prophet, and prophecies can only come true if others believe it.

Like one user previously stated, Kenpachi's arrival is probably due to Gremmy's power. Yachiru always believes that Kenny will come save her when she needs it, and lo! he shows up right before she dies. I don't believe he's an illusion though, as Yachiru wouldn't know that Unohana is dead (since she wasn't there when it happened).