I was playing Arena and was offered this card and the two other legendaries were even worse. My run finished 3-3 so six games. The “random anomaly” has triggered 0 times.
Does Cho’gall actually work in Arena? Why such useless card is actually offered to somebody when others get proper legendary to build the deck around?
This is a current bug.
While it is a bug, its still possible to go 3+ games without it activated, you can flip a coin a thousand times and get tails every time, not likely, but it could happen
11 days later and it happened to me as well. I’ve played 6 games so far and the anomaly has never triggered. I say either make the card work or remove it from the arena draft pool. I’m not mad that the legendary is bad i just wanted anomalies thought it would be fun. Disappointing 
i just want to go off of what fuzzy said, you could go all 11-14 games without it ever working
but its probly still bugged