With this dagger (and another attuned weapon in your main hand slot), you have a small chance of doing 25% extra damage on a hit. This chance varies with your Agility level and maxes out at 12% at 99 Agility. So at most, this gives it a 3% damage boost over other tier 80 weapons.
The off-hand attuned crystal dagger will revert to seed after 50,000 charges of combat, but uses two charges per hit except while within Prifddinas, where it uses one charge per hit. The amount of coins needed to repair a partially degraded off-hand attuned dagger is scaled linearly to your current creation cost of a normal crystal weapon/armour seed. This reduces with each creation, starting at 1,000,000 coins, and decreasing by 200,000 with each creation, until five or more creations - where a creation/full repair will costing 200,000 coins. However, the off-hand attuned dagger cannot be repaired for coins if it is fully degraded, as it reverts to seed and has to be re-created with harmonic dust.
This weapon can be stored using a keepsake key, but only when newly made/repaired - used versions will need to be repaired first.