"subtitle"
https://backtothethames.files.wordpress.com/2014/10/inigomontoya.jpg
That quibble aside, I agree. "The Void Burrower" sounds pretty dull. That might work if this were a literal subtitle, or perhaps spoken narration for visually impaired viewers ("a Void Burrower attacks"), but it's a very literal way to refer to the Champion, and strikes me as strange-sounding.
I'm reminded of Orianna being changed from "The Clockwork Girl" to "The Lady of Clockwork" due to some alleged IP concern. (I think?) I thought that was a weak change. (As one friend observed to me, even "The Lady Clockwork" would have a better sound to it.)
I recall reading that there are two main schools of thought in Riot about Champion titles: some think they should be sort of poetic and abstract, while others thing they should be very literal. The example given was Skarner, as "the Crystal Vanguard"; one Rioter said that if it had been up to them, he'd be called "the Crystal Scorpion".
Personally, I prefer the more descriptive titles, as I think you do as well. For instance, I think it's cooler to have Garen known as "the Might of Demacia" than "the Demacian Warrior" or "the Armored Knight" or something.