Decimation shards are dropped by creatures in the second floor of the Monastery of Ascension. They carry many uses in the game. Firstly, 100 shards are required to enchant an Ascension crossbow into a Decimation crossbow along with a few other items. Secondly, another 100 shards can be used for turning a Decimation crossbow into it's off-hand counterpart by talking to Ocellus. They may also be fletched into decimation bolts.