You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect. If it is casting a spell of 4th level or higher, make an ability check using your spellcasting ability. The DC equals 10 + the spell's level. On a success, the creature's spell fails and has no effect.
At Higher Levels. When you cast this spell using a spell slot of 4th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used.
Spell Mastery. A DM may reward a spellcaster with a spell mastery - a spell with which they have such prowess that the spell gains additional or alternate effects. Perhaps through sheer frequency of use, the player has developed an innate understanding of how the spell can be manipulated. Perhaps they spent extensive downtime in research or under the tutelage of a powerful mage. Perhaps a spell scroll with lost knowledge of the spell's potential was found in a dragon's hoard. However it is acquired, a spell mastery defies bounded accuracy and should not be granted liberally. A DM may grant one or more spell mastery options for a given spell, but should grant no more than one spell with spell mastery per tier of play.
Spell Mastery 1: You have advantage on spellcasting ability checks to successfully counter a spell whose spell level is greater than the level of spell slot used to cast this spell.
Spell Mastery 2: So long as their is a clear line of sight between you and the target, the target no longer benefits from effects that would obscure their casting of the spell, such as the Subtle Spell Metamagic feature.
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