“Venomancer” was used prior to Dota; it’s a fairly common gaming term for a poison-using mage. It’s a generic combination of “venom” and “-mancy,” a productive suffix in English that just means “type of magic.” Dota was far from the first to use that particular combination. The term was used (interchangeably with “poisonmancer”) in the Diablo II community as a term for poison-focused necromancers, for instance—and that wasn’t the origin of it, either, I’m sure.
Even in the Dota context, IceFrog got the character from Guinsoo’s Dota All-Stars, who got it from Eul’s original Defense of the Ancients.
This matters because intellectual property law often cares about “prior art,” and the Venomancer character—even specifically the Dota one—isn’t really Valve’s. Warcraft III map-makers were constantly stealing each other’s work, and there’s no way that Valve tracked down everyone who had a hand in this and purchased whatever rights they had to things. Eul and IceFrog work for Valve now but Guinsoo works for competitor Riot, for example. And it’s not as if any of this was chronicled or recorded terribly well. And beyond the specific Dota character, the term “venomancer” is so generic—and so widely used—that no one can have any rights to it.
So if you write a character who uses poison magic and is called “venomancer,” or even “The Venomancer,” you’re really just using a word. An uncommon one, but by no means necessarily specifically reference to any given character, even if Dota’s is the most well-known at this point. After all, the Dota Venomancer has a name (Lesale Deathbringer); “venomancer” is just a title. And even if it did, Valve would struggle to defend their claim to that character anyway.
Whether or not you should is a separate question, though. A large portion of your audience might immediately think of the Dota character, which may not be what you want.