Mob Essence is a greenish-colored liquid 
produced by the Grinder or extracted from a player's own experience levels by using an XP Extractor. It can then be consumed in an Auto-Spawner to create specific mobs, piped to a Block Smasher to process ores with a fortune enchantment, or used to enchant items in the Auto-Enchanter. Swimming in a pool of Mob Essence grants the night vision potion effect for a full minute, which is especially handy for underwater exploration.
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MineFactory Reloaded - Mob Grinders & Auto-Spawners