Charge-up Skills are attacks that add a "charge" for each successive hit within a short time frame. While each Charge-up attack deals normal damage, the charges continue to accumulate until triggered or until the time limit runs out.
To trigger the accumulated charges, strike a monster with either a normal attack or a Finishing Move. Different Charges can stack, meaning that one can maintain charges from multiple skills provided they refresh each stack of charges before they time out.
The Assassin can mix and match charges, for example:
You attack a target twice with a level 13 Tiger Strike gaining two charges, using a Finishing Move now will give you +680% Damage. If you instead attack an enemy with a level 11 Cobra Strike gaining 1 more charge for the maximum of three, then attack with a Finishing Move you will now gain +680% damage and 90% life stealing.
Power-up skills of Druids work slightly differently: each charge empowers the skill it uses, making next cast stronger. There's no need to trigger it: the skill simply grows in power when continuously used, stacking up to 5 times, but charges still fade if the skill is not cast within reasonable time. Unlike Assassin's Finishing Moves, Power-Up casts do not consume charges when used.
- Charge-up mechanic for Assassin and some Druid skills was later used by Blizzard in World of Warcraft as a Combo point mechanic for Rogue class. It works exactly as in Diablo II: some skills accumulate charges, while others consume them for empowered effect.