Circa. 2,600 BC, Lev was joined to a young male host and served as a science officer in the Trill military during the Kurlan genocide. He was a bridge officer aboard a Trill starship that was enforcing the medical quarantine at Kurl. When sensors detected four Kurlan ships breaking orbit, he informed his commanding officer who ordered the weapons officer, Private Memh, to destroy the ships. Reluctantly, Lev then informed the captain that more Kurlan ships could still be launched and that they'd never be able to destroy them all before they escaped. The captain then ordered the release of biogenic weapons and incendiaries on the planet's surface, to prevent any further launches.
In the year 2376, Ezri Dax learned of the Kurlan genocide, and Lev's part in it, through Memh's memories which were shared with her by Annuated deep in the Caves of Mak'ala. (DS9 novel: Trill: Unjoined)