These are the lyrics to a metal song about Lovecraft's The Dunwich Horror. The band tends go Classical for effect (cf. the refrain at the end: 闇にこぞりて / 我が主来ませり …). The first verse is transcribed in most Japanese sites as this:
However, when I listen to the song, I hear the singer go /o.so.wa.ra.ta/ (0:56- here – notice that his /r/ is often lateralized; cf. 0:41, 0:54, 3:24 etc.). Not osowatta, not osowareta, but osowarata.
When I told my teachers about this they said, “B-baka na! Surely you're mishearing! Sonna koto wa nai hazu! That form doesn’t exist, it doesn’t even make sense!” They said it was impossible. They said no native speaker would ever produce it. But I'll show them! With the dark powers of ad-hoc Google searches, I'll show all of them!
From 『去日来日』 by 植松正 (1977)：
From 『逸見東洋の世界』 by 臼井洋輔 (1990):
From 『現代のエスプリ』 , somewhere between nos. 286 to 289:
These 3 are all we get from Google Books, but they're in actual print. Google Scholar claims two more hits but they're restricted, and a regular web search gets some ~20 examples (versus 170 for osowatta).
So my question is: Are these all typos? A surface phonetic process (which)? A dialect form? Language change? And what does it mean – is it equivalent to osowaratta、osowareta (if this even makes sense?)、 some Classical form I don’t recognize, or what?