This is a new promotion from Castle Age : Players have to collect stones to earn a new Guild Class Ability.
The stones are a new gift item, available only for 8 days. From Dec 02 to Dec 09, 2010.
To get the maximum reward, you need to collect 100 stones. Its similar to the Golden Egg promotion from a while back so even if one does not have 100 stones at the end there will be some lesser rewards for intermediate stone counts.
Reward levels:
10 Powerstones - Might Level 1 (+1 Attack and +1 Defense in Guild Monsters and Battles)
25 Powerstones - Might Level 2 (+2 Attack and +2 Defense in Guild Monsters and Battles)
50 Powerstones - Might Level 3 (+3 Attack and +3 Defense in Guild Monsters and Battles)
100 Powerstones - Might Level 4 (+4 Attack and +4 Defense in Guild Monsters and Battles)