(covers information from several alternate timelines)
The Zahl were a technologically advanced but non-confrontational and friendly humanoid species native to the Zahl homeworld, in Grid 005 of the Delta Quadrant. The Borg rated their resistance quotient to be "quite low".
In one possible timeline, a (Zahl) generation prior to 2374, the Zahl defeated the Krenim Imperium, despite the temporal science they used to construct advanced weapons. They disbanded the Krenim fleet and retook the territories they had annexed. By the 2270s, they were the primary species in Grid 005, while treating the inferior Krenim ships still moving around making grandiose claims about the territory being disputed as a minor nuisance.
In 2374, two Zahl starships greeted the USS Voyager upon its arrival in their space. During the meeting, the entire Zahl species was erased from history by Annorax's weapon ship. The erasure of the Zahl achieved an unprecedented 98% Krenim restoration, although the Krenim colony on Kyana Prime was not restored. When the weapon ship was destroyed, this timeline was negated. (VOY: "Year of Hell")
In the novel A Pocket Full of Lies, the crew of Voyager and the Full Circle Fleet (β) investigated disputed space of the Zahl and the Rilnar following the discovery of the survival of the alternate Kathryn Janeway encountered by Chakotay during the incident in "Shattered" in which the USS Voyager was shattered into 37 different timeframes. The novel reveals that the Zahl and the Rilnar are two tribes of the same species, which evolved on the planet Sormana.