The 13-Day Fake Rule Deception
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Ryuk's Forged Constraint · A Temporary Alibi
Light has Ryuk add false rules to the notebook recovered from Higuchi. The most important claims that an owner will die if thirteen days pass without another name being written. Because Light and Misa were confined for longer than thirteen days while the killings continued, the rule appears to prove they could not have been Kira.
What changed
The fake rule buys Light an alibi, not permanent safety.
L Never Fully Accepts the Thirteen-Day Rule. Once he moves toward testing it, Rem understands that Misa can become a suspect again.