This was mentioned in 1/23 (Wed) Update Content

Stages [4. Weeping Desert] and [10. Eastern Outskirts of Tirel Castle] allow players to select their difficulty level. Players may only select a higher difficulty level if they have cleared the one before it. Remember, the higher the difficulty level, the stronger the enemies, and the better the rewards!

1. Regarding Rewards:
   1. Reddit Link to Perland an Analysis from the Event
   2. According to the reddit, the amount of experience per energy is less than the normal as difficulty increases and should be increased for the higher difficulties.
   3. It is unknown, the drop rates of units / gold / catalyst / item to begin with and how these drop rates change with the difficult level. The effective rate of the catalyst should be at least as high as the effective rate and hopefully slightly better to account for the fact that there is only one drop per battle limitation.
   4. Decrease the amount of experience required to upgrade event artifacts. Enhanced artifacts do not scale appropriately with the required investment, unlikely to see much use outside of the event itself.
   5. lvl 6, 8, 10 artifacts should be considered to be part of the next level rather than the previous level (lvl 6 is currently +15% should be +25%)
2. Achievement Costs
   1. Change "Clear battle" stages to "energy used" used stages.
   2. For 150 battles at (Easy)   8 Energy per battle requires 1200 energy. (no epic catalyst chance)
   3. For 150 Battles at (Hell) 30  Energy per battle requires 4500 Energy.
   4. Increase wave of enemies with difficulty
   5. For eliminating 1500 monsters, the cost of energy for Easy is expected to be 1504.
   6. For eliminating 1500 monsters, the cost of energy for Hell is expected to be 5640.