I had to go look up the game script to try to figure out what I knew from the game and I had made up for my own stories. I fond this:

Knoll:

One year ago,t he emperor died of
the illness that had plagued him.
Prince Lyon was devestated. He
fell into an endless despair.
He felt unworthy to take the throne,
unable to rule without his father.

[...]

Prince Lyon told us he intended to use
the Fire Emblem to restore the emperor.
His death had been kept from the public,
but rumors were beginning to fly.
We knew we had to hurry, before the people
learned the truth of the emperor's death.

Sounds like they just said he was bedridden for a while and then, once he was reanimated, they said he was on the mend. I'm trying to imagine Lyon reanimating a year-old corpse.

*shudders*

He was clearly off the deep end and in the clutches of the Demon Lord by that point.