Monster Hunteralways had various hidden mechanics regarding how attack is determined, the percentage of drops and much more. RecentlyCapcomhas published miniature guide books explaining these various hidden mechanics that affect players. Unfortunately, they are only available in Japan at the moment with no word if any official English version will be available.
Most players have reached or in the process of reaching the end game portion. and there is still lots of confusion surrounding it. So let’s attempt this one step at a time.

Table of Contents
What defines end game?
Tempered Tier List
Tempered Quest Rewards
Official Tempered Quest Loot Percentages
Official Decorations Category Percentages and Classification
Official Decorations List
The End Game
The end game consist of being Hunter Rank (HR) 50+ which players spend a majority of their time, finding tempered monsters tracks to obtain tempered monsters investigations to obtain loot. This loot consist of Feystones which are hidden decorations (gems) that can be place on weapons and armor if they have the proper slot and Stones which allow players to augment their weapons and armor.
Most players are unsure which monster falls under which tier and its pretty easy although kinda backwards. The quest details state what HR players have to be and using that information, one can determine which tier it falls under. However, this trick only works once a hunter has a quest so I have listed each monster and which tier they fall under.

HR 13+, it falls under Tier 1
Tier 1 have common monsters.
HR 30+, it falls under Tier 2
Tier 2 have apex monsters and other late game monsters.

HR 50+, it falls under Tier 3
Tier 3 are the easiest to understand at the moment since its only Elder Dragons
After a successful tempered quest, hunters can obtain some loot consisting of:
Official Tempered Quest Loot Percentages Pre-Patch 3.00
Threat 1: 60% Mysterious Feystone, 35% Gleaming Feystone, 5% Worn Feystone
Threat 2: 36% Gleaming Feystone, 35% Worn Feystone, 26% Warped Feystone, 3% Warrior Streamstone
Threat 3: 72% Worn Feystone, 13% Warped Feystone, 12% Warrior Streamstone, 3% Hero Streamstone
Tempered Quest Loot Percentages Post-Patch 3.00
Threat 1: 55% Mysterious Feystone, 25% Gleaming Feystone, 15% Worn Feystone, 5% Warped Feystones
Threat 2: 34% Gleaming Feystone, 35% Worn Feystone, 26% Warped Feystone, 3% Warrior Streamstone, 2% Hero Streamstones
Threat 3: 70% Worn Feystone, 13% Warped Feystone, 12% Warrior Streamstone, 5% Hero Streamstone
Each decoration will fall under a tier consisting of: C , B , A , S with C being the most common and S being most rare.
*unconfirmed but I believe within each tier there is a tier in that. Meaning while C is the most common there are still “less” common gems hidden there.