How to understand 'quae prosum sola nocendo'?

Question

There is a line in Ovid's Metamorphoses II 519, which I don't understand at all

(Juno's complaint)

'quaeritis, aetheriis quare regina deorum
sedibus huc adsim? pro me tenet altera caelum!
mentior, obscurum nisi nox cum fecerit orbem,
nuper honoratas summo, mea vulnera, caelo
videritis stellas illic, ubi circulus axem
ultimus extremum spatioque brevissimus ambit.
et vero quisquam Iunonem laedere nolit
offensamque tremat, quae prosum sola nocendo?
o ego quantum egi! quam vasta potentia nostra est!<

Nocendo is gerundive from noceo — "to do harm, inflict injury, hurt" — and prosum is a verb "to be useful, be of use, do good, benefit, profit, serve".

This line is translated as 'who do but help where I would harm?' (F. J. Miller)

The gerundive (passive adjective) would be then 'to be harmful towards something, (requiring, needing, must be harmed) and should usually have the agent in the dative — here perhaps sola?

And quae can be f.nom., f.Pl.nom., or n.Pl. nom., acc. — what or who (as it cannot be Iuppiter?) does it refer to, I do not know.

I assume prosum means then that she is saying 'I do good'…

Answer 1

Nocendo is a gerund (noun) here, not a gerundive (adjective). Therefore, it's active in meaning. It's ablative to show the means by which Juno does good. Quae is f. nom. sing. and refers to Iunonem, who is the 'I' of the relative clause.

...I who alone do good by doing harm

(Note that sola couldn't be the object of nocendo (or the agent, because nocendo, as a gerund, isn't passive in meaning), because it's not dative; the form would have to be soli. In this case, nocendo is being used 'absolutely' – that is, without an object; she's talking about doing harm in general.)