Each mission in Castlevania: Lords of Shadow has an associated 'Trial' challenge. These are optional objectives that, when completed, reward you with a 'Trial Gem'. Collecting all Trial Gems is necessary for achieving 100% completion and unlocking certain achievements. Trials can range from completing a mission without using magic, defeating a certain number of enemies with a specific attack, or finishing a mission within a time limit. They often require a deep understanding of the mission's layout and enemy patterns. Replaying missions with the intent of completing a trial can be a great way to hone your combat skills and discover new strategies. Don't be discouraged if you don't complete a trial on your first attempt; they are designed to be challenging and encourage mastery of the game's mechanics. Check the mission select screen to see the specific trial for each mission.
