Indexes are used to complete collections. You can obtain Harvest Fields indexes by killing its corresponding monster in Harvest Field. The Harvest Fields are home to the most vast amount of quest mobs so far, giving rise to many indexes and subsequent collections.
Classic
Village Collection
Required Indexes:
Thug, Blighted Wheat, Edwin
Collection Effects:
10 Endurance
Thug Collection
Required Indexes:
Thug, Outpost Thug, Roaming Thug
Collection Effects:
10 Dexterity, 8 Strength
Forest Collection
Required Indexes:
Outpost Thug, Roaming Thug, Lt. Roth
Collection Effects:
10 Strength
King's Outpost Collection
Required Indexes:
King's Templar, Hammer and Anvil, Bandit King, Deadeye Eirik
Collection Effects:
10 Intelligence, 5 Perception, 5 Dexterity, 5 Strength
Mansion Collection
Required Indexes:
Hound, Vex, Gorth, Bronto
Collection Effects:
12 Dexterity
Fortress Collection
Required Indexes:
Savage Bandit, Tilan's Guard, Markus Tilan
Collection Effects:
50 Vitality, 5 Strength
Harvest Minibosses Collection
Required Indexes:
Edwin, Lt. Roth, Vex, Gorth, Bronto, Garrold
Collection Effects:
5 Intelligence, 20 Perception
Harvest Boss Collection
Required Indexes:
Hammer and Anvil, Bandit King, Deadeye Eirik, Markus Tilan
Collection Effects:
Harvest Fields Collection
Required Indexes:
Thug, Outpost Thug, Roaming Thug, King's Templar, Blighted Wheat, Hound, Savage Bandit, Tilan's Guard, Lt. Roth, Edwin, Garrold, Vex, Gorth, Bronto, Hammer and Anvil, Bandit King, Deadeye Eirik, Markus Tilan
Collection Effects:
100 Vitality, 20 Strength, 20 Intelligence, 20 Dexterity
Shiny
Shiny Thug Collection
Required Indexes:
Shiny Thug, Shiny Outpost Thug, Shiny Roaming Thug
Collection Effects:
Shiny Village Collection
Required Indexes:
Shiny Thug, Shiny Blighted Wheat, Shiny Edwin
Collection Effects:
Shiny Harvest Minibosses Collection
Required Indexes:
Shiny Edwin, Shiny Lt. Roth, Shiny Vex, Shiny Gorth, Shiny Bronto, Shiny Garrold
Collection Effects:
Shiny Harvest Boss Collection
Required Indexes:
Shiny Hammer and Anvil, Shiny Bandit King, Shiny Deadeye Eirik, Shiny Markus Tilan