In April 2017 King of Avalon was extended with Noble Chests as rewards for slaying monsters.
As a reward for slaying monsters you get a Noble Chest. Like the Barbarian Bonus chests, a Noble Chest can be of different level, 1 to 4. I have so far only slain lvl 25 and 26 Monsters which resulted in Noble Chest lvl 3, and gotten lvl 4 Noble Chests from the Barbarian Chest Lvl 6. I am not sure whether you can also get Noble Chest lvl 4 from higher level monsters, and when you get lower level Noble Chests.
So far for each (lvl 25 and 26) Monster that I have slain since this feature was introduced I received the Noble Chest.
For the Daily Rewards for Prize Stages 3, 5 and 7 you now receive an additional prize, the Noble Key. It was already important to complete the Daily Rewards uptill prize stage 6 because of the Barbarian Key, now the Noble Key also gives an extra motivation to complete prize stage 7 as well.
As far as I have seen so far this is the only way to get Noble Keys. Average players are very likely to accumulate more tha 3 Noble Chests a day, so I expect everyone building up a large collection of unopened Noble Chests.
You can open the Noble Chests by selecting them in your items and Synthesizing them with Noble Keys. You need one Noble Key for each Noble Chest. Synthesizing leads to Unlocked Noble Chest in your inventory which you can Use.
Since you can get Noble Chest lvl 4 by beating lvl 26 and up Barbarians with your alliance mates (which should be possible for most players in large alliances, even if you are not very large yet), you might hold on to your Noble Keys to only open Noble Chest lvl 4. Although the prizes I have received for the lvl 3 and 4 Noble Chests so far are quite similar.
The lvl 3 Noble Chests I opened so far gave me the following prizes: 30k Iron and 150k Food.
The lvl 4 Noble Chests I opened so far gave me the following prizes: Noble Badge (very rare), Small Lord EXP, 3h Speedup, 30k Iron, 150k Wood and 150k Food.
Each Chest contained one prize.
If you have any questions, or comments, or if you can share confirmed Noble Chest levels for other Monster levels, or prizes from the chests, let me know in the comment section.