Is this grammatical in English?
An Italian wizard named Mirkius has found, and managed to control, a ferocious creature named Onestus
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An Italian wizard named Mirkius has [found], and [managed to control] a ferocious creature named Onestus.
Yes, it's grammatical. Some grammars call this a 'delayed right constituent coordination'!
In this construction, the constituent which in basic coordination would appear as the rightmost element of the first constituent is held back until after the final coordinate. The basic coordination would be:
An Italian wizard named Mirkius has found a ferocious creature named Onestus and managed to control it.
In the delayed constituent analysis, each coordinate allows completion by the delayed element: "found" and "control" both allow completion by a noun phrase complement.
The term 'parenthical' doesn't seem appropriate here (the commas aren't necessary), though typically there is a prosodic break after the final coordinate, signalling that the element that follows relates to the whole coordination, not just to the final coordinate.
Yes, it's fine in English. It's not "parenthetical" though. It is a right node-raising construction, so called after Paul Postal's proposal for its analysis. See Right node raising. The raised node is "a ferocious creature named Onestus", which belongs logically in two places in the preceding part of the sentence, at the right of the two conjoined constituents.
The analysis is disputed. James McCawley proposed that the "raised node" is simply shared by two mother nodes in the tree structure, and this structure is not a strictly hierarchical tree.