Cleaver is one of the Heroic Chest's grand prizes. Therefore, his drop rate is relatively low compared to other heroes.
A guaranteed grand prize drops every 400th Chest opening. However, you can still get the grand prize earlier. The chance depends solely on luck. Cleaver is always the first grand prize.
The counter below the grand prize shows how many openings remain until the prize drops.
Once you get Cleaver, a message about it will appear in the server chat, and the Heroic Chest grand prize will change to either 50,000 Hero Soul Coins or one of the Skins+. Their drop chance is the same, and you can get either.