Thaumcraft Treasure Bags

The mod Thaumcraft adds three treasure bags: Common Treasure, Uncommon Treasure, and Rare Treasure. They are obtained by killing Thaumcraft's Champion mobs. Right-clicking opens them, which gives a variety of Vanilla and Thaumcraft-related resources.

Champion Mobs
Zombies, spiders, blazes, endermen, and skeletons have a base chance of 1% of spawning as champions, and witches 2%; if the world difficulty is hard, this gets bumped by 2%, in the outer lands this gets bumped by 3%, and in biomes with attributes SPOOKY, NETHER, and END there's a 2% bump.

Obtaining Treasure Bags
Killing champion mobs drops treasure bags. Using a weapon with a higher level of looting increases the rarity of the treasure bag. With Looting III, rare treasure bags may drop, with a 1 in 9 chance of dropping.

Besides killing champion mobs, Kobolds from The Twilight Forest drop common treasures, and Thaumcraft bosses drop rare treasures.

Loot
Common and uncommon treasures typically give coins. They may also give several types of potions, including splash potions of weakness. Rare treasures may also rarely give nether stars and primordial pearls, with a chance of 1 in 2651 for each.