A legendary Tavern is an event in Rise of Kingdoms that is available during Season of Conquest. Legendary Tavern is decent and you should be able to get a nice amount of Legendary commander sculptures but the biggest problem with this event is that P2W players can get legendary commander Sculpture even easier.
To open one chest from Legendary Tavern you will need one Sovereign Key that you can obtain in 3 different ways. 2 ways are pay to win and one is from KVK achievements.
Pay to win players can open an insane amount of these chests and with that, they will have a lot of max commanders. The limit is 2000 chests per day and that is a lot.
When it comes to free-to-play players you can use your hard earned gems to open the chest but it is not worth it. It is better to spend your gems on more than a gems event or on wheel.
When it comes to opening this chest it is better to open 10 at the same time because one legendary commander is guaranteed. That means that you should avoid opening one by one.
When you open 200 Legendary Tavern chest you can choose what reward you want.
If you open Legendary Tavern chest and get legendary commander that you have maxed out you will get coins back that you can use in Legendary Tavern shop to buy rewards or one more chest.
Legendary Tavern events could be great if there was no pay to win purchases.