As a part of Riot Games' Mythic Content Overhaul, a variety of things have been changing. Masterwork Chests are included in this, and in the recent Patch 12.7, they received adjustments in what rewards rewards you can receive from opening them.

League of Legends Masterwork Chest Rewards

Opening a Masterwork Chest itself has a chance to contain a skin shard, an emote, orange essence, or a ward skin shard. You have a 70% chance of getting the skin shard, and a 10% chance each of getting the emote, orange essence, or ward skin shard. Alongside this, each Masterwork Chest has a 3.5% chance to give you five Mythic Essence, and a 0.04% chance for something loot exclusive. If you want to read more about the introduction of Mythic Essence, check out the 12.6 patch notes.

As a part of Riot's Mythic Content Overhaul, something new was introduced to Masterwork Chests in Patch 12.7. There are now repeating rewards for opening Masterwork Chests. For every five Masterwork Chests you open, you will be given a reward, with the value of each increasing the more Masterwork Chests you open.

For every five Masterwork Chests you open, you will receive five Mythic Essence. There's more than just this: you will receive a skin shard of any quality for opening five Masterwork Chests; for opening 15 Masterwork Chests you will receive a skin shard for a skin worth more than 975 RP; and for opening 25 Masterwork Chests, you will given a skin shard for a skin worth more than 1,350 RP.

These changes encourage players to buy more Masterwork Chests to reap more possible rewards. Masterwork Chests cost 165 RP; purchasing 650 RP costs $5.