Earlooms are items required to level up a guild from 4 to 5 as part of the task to get the Elder's Recommendation - one of the items needed to level up. To acquire this recommendation, each guild gets a specific castle target and a variable amount of earlooms needed to be obtained from this castle.
Example of task message:
To prove me your worthyness for my recommendation, bring me 49 butterfly earlooms.
During the Battle of the Seven Castles, members of the guild have to attack the specified castle (in this case: butterfly castle). For each member that successfully attacked the specified castle, an earloom will be rewarded in the guild stock (
/g_stock_misc). Each member can only obtain one earloom per battle. If the member did not managed to attack the specified castle successfully, no earloom will be rewarded for that member.
Currently[as of 2018-06-05], there are only six types of earlooms, as Moonlight Castle seems to be excluded from the possible targets.