Enervation, a 5th level spell from Xanathar’s Guide to Everything, has the following text:
The target makes a Dexterity saving throw. On a successful save, the target takes 2d8 necrotic damage, and the spell ends. On a failed save, the target takes 4d8 necrotic damage, and until the spell ends, you can use your action on each of your turns to deal 4d8 necrotic damage to the target.
When you cast this spell using a spell slot of 6th level or higher, the damage increases by 1d8 for each slot level above 5th.
First, it seems clear that all damage is increased by 1d8. For instance, Witch Bolt only increases the initial damage by 1d12, and with Ice Knife, the cold damage is boosted, but not the piercing damage.
Now, suppose I cast Enervation at 9th level and the target succeeds its DEX save. As written, the target takes 6d8 damage. Very unusual. Normally, I’d roll ALL THE DICE (8d8 in this case), and then halve the result for a successful save.
But speaking of halving the results, consider the Evasion feature for the Monk and Rogue classes:
When you are subjected to an effect that allows you to make a Dexterity saving throw to take only half damage, you instead take no damage if you succeed on the saving throw, and only half damage if you fail.
So: when does Evasion work against Enervation? Obviously not at 6th level or higher, but will Evasion activate when Enervation is cast at 5th level? Is 2d8 considered half of 4d8?