SAV Great Building - How can I obtain a mysterious shard through the Flying Island?

The mysterious shard is one of the six rewards you can get from a rare shard incident. The Flying Island will spawn incidents in your Cultural Settlements, which can be collected from time to time to obtain some great rewards.

If you get a mysterious shard in the Vikings or Japanese settlement, you will receive 300 random goods of your age. If you get the mysterious shard in the Egyptians or Aztecs settlement, you will receive 150 forge points. 

You can find more information about potential shards from the Flying Island's Great Building in the table below:

Common	Uncommon	Rare
20 forge points 25 random goods (current era) 25 random goods (previous era) 1 or 10 light units 1 or 10 ranged units 1 or 10 heavy units 1 or 10 artillery units 1 or 10 fast units 2 blueprints coins supplies	100 forge points 200 random goods (current era) 100 random goods (previous era) 10 rogues 8 hour rush mass coins 6 hours rush mass supplies mass self aid kit	5 blueprints medals one up kit renovation kit store building Mysterious shard