Powerdown went away in July 2014.
The FAQ needs to be updated then: http://www.openbsd.org/faq/faq10.html "rc.shutdown/etc/rc.shutdown is a script that is run at shutdown. Anything you want done before the system shuts down should be added to this file. If you have apm, you can also set "powerdown=YES", which will give you the equivalent of "shutdown -p".
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