This is a play from the 14th century (reportedly "the first secular tragedy written since Roman times") relating the tale of Ezzelino da Romano and written by one Albertino Mussato. Is there an online English text? Or at least a published book where it appears in translation?
There appears to be an out of print version put out by Penn State in 1972. If you can't locate a copy of that, you could try the physical library at Penn State.
This is the kind of thing that really needs to be made available at Project Gutenberg, but somebody has to volunteer to do the work.