I don't know what level it procs, But I can narrow it down slightly. A lvl 37 Bard friend of mine was
unable to use it.

Talies the Wanderer wrote:

> Well, you guys asked me to let you know what I got for the Horn in the
> end. To bring you up to date, I lucked into a Horn of Hsagra, doing a
> low-level quest in Ak'Anon (probably due to a loot-table mixup). I offered
> it to a friend of mine who is in a guild, and he said to write my own
> ticket. So I did. I requested the Lute of the Howler, and some coins for
> the Nostrolo Tambourine quest. I didn't expect to get all of that, but I
> was hopeful.
>
> Today I got a call, requesting my presence in West Commons. There, I was
> presented, in trade for the Horn, a bag containing one Lute of the Howler,
> 6 Ancient Coins, and 18 peridots.
>
> I'd say I did well.
>
> Talies the Wanderer
>
> P.S. What level can I use the Lute's illusion?
>
>
> Please send submissions for the eqbards newsletter to lol@... with the subject submissions.
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/