Level 10 Normal Boss : A griffin coming from the outland, very merciless and evil
The LVL 10 boss is the Griffin. He has a 17.9M power and is classified as a medium monster. He will use 20 stamina for an attack. It is suggested by Evony that you can beat these bosses with 250,000 T10 cavalry troops (200,000 T11 for no wounded). I did come across another reference that indicates you can kill him with 130,000 T9 cavalry, but it does give a high (9000) wounded rate. I would think that you would need a really good general and buffs to get results like that.
The rewards for killing a Griffin include the level 10 boss chest (contains medals, general exp,9 material chest and a speed-ups), refining stones, spiritual beast exp, artwork fragment chest, an epic (LVL 5) treasure map and the new chance of getting a King's Scroll Chest (contains scrolls to craft Kings Equipment). The added bonus to everyone in an alliance is that they will all receive a Senior Alliance Boss package every time a Griffin is killed by an alliance member, (and just to remind you of the contents - which contains 50K food, stone and lumber; training and healing speed ups and a material chest).