Well, there are two possible champions, in our mind, that are coherent enough to actually say these words. After all, most of the voidlings can't even speak.
In today's challenge, the quote of the day is: "Bow to the void. Or be consumed by it.". It immediately points to champions that are allies of the Void or are from the Void itself. It certainly wasn't said by the various Darkin that's for sure!
The answer to the question of which champion in League of Legends says: "Bow to the void. Or be consumed by it", is Malzahar.
Malzahar used to be a seer in the cities of the ruined Shuriman empire. There, after his life didn't amount to much, and seeking wisdom, he traveled to Icathia, where his mind was corrupted by the Void.
Now, he acts as the Void's most valuable agent on Runeterra, that is, the Prophet of the Void.
He is the sole reason why Bel'Veth has managed to enter Runeterra and consume the city of, well, Bel'veth, after which she was named. He is the arch nemesis of Kassadin, who uses deadly ancient artifacts to hunt him down, for killing his family and destroying his village.
What we want to know, though, is why Malzahar, with his reputation, was then given a skin that celebrates honorable LoL players.