Creeper Shards are powerful, everlasting items that are dropped by the Creeper Egg if it doesn't drop 5-10 gunpowder.
The Creeper Shard can be used as a weapon in two ways: you hit mobs with it, dealing 2 damage and decent knockback, or you can left click to cause an explosion. The explosion's power level is based on the Creeper Shard's 'durability', which is set to 0 upon left clicking with it. Every time you hit a mob with the shard, it increases the durability by 1 per hit, recharging it. As a consequence, there is a 10% of you becoming poisoned every time you land a hit on an entity.