One of the three main types of items commonly found in stages alongside treasures and food, the Jars are restorative pickup items used to replenish the protagonists's ability to use secondary items. Enemies are insured to, eventually, drop jars. Any jar that was not originally out in the open when first found (for example being dropped by enemies), will eventually blink out of existence if not collected quickly. Any previously collected jar who had a fixed location (out in the open, found in dirt...), will respawn upon death where it was originally found. Their appearance vary with each campaign:
In Shovel of Hope, Shovel Knight can pick up Magic Jars to restore specific values of his numerically expressed Magic Capacity, which is used to activate Relics. Small Jars restore 5 magic whist the large ones restore 30 magic. Magic Jars are found commonly throughout the game by defeating enemies, breaking blocks or at specific spots in stages. The use of an Ichor of Fortune will magnetize any jar in range towards Shovel Knight.
The Magic Capacity can be further increased by buying Magic Drinks from the Magicist.
In Plague of Shadows, Plague Knight can pick up Power Jars to restore part of his Power Meter, which is used to cast Arcana. They appear throughout the game in the same way Magic Jars do. Small Jars restore half a portion of Plague Knight's Power while a large one restore two portions. Unlike Magic, Power naturally regenerates over time. Using the Treasure Trappings will magnetize any jar in range towards Plague Knight.
The Power Meter can be further extended by buying Magic Upgrades from the Magicist.
In Specter of Torment, Specter Knight can find Darkness Jars to restore a portion of his Darkness Capacity, which is a meter used to power Curios. They appear throughout the game but they also drop and home in towards Specter Knight in the form of little tufts of purple flames when he defeats enemies or sufficiently hurts the more bulky ones (similarly to the red orbs generated by the use of the Armor of Chaos). For the darkness flames to appear the enemy must have been damaged at least once by Specter Knight as, if the enemy is defeated without any direct intervention from him (for example falling down a pit on it's own accord) no flames will appear.
Small Jars and flames restore half a portion of Specter Knight's Darkness while large Jars restore three portions. Candles with purple fire, usually found in the Tower of Fate, generate an endless source of Darkness flames.
The Darkness Capacity can be further extended by finding Darkness Wisps hidden within the Explodatorium, the Clockwork Tower and Lich Yard or by buying them from Missy, if failed to be located the first time.
In New Game Plus, Specter Knight's Will and Darkness are fused in one ever-depleting meter, and as such Darkness Jars and flames act at the same time also like a food source, and passing for the first time by a Checkpoint will completely refill Specter Knight's fused "Darkness+Will" gauge.