Deny

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 1st level or lower, its spell fails. The spell slot used and any material components the spell might have required are not consumed/used. If the creature is casting a spell of a higher level, make an ability check using your spellcasting modifier. The DC is equal to 10+ the target's spellcasting modifier. On a success, the spell fails.

When attempting to deny a spell that was cast using a higher level spell slot than you used, you take 1d4 times the difference in level of spell slots used as necrotic damage. 

Example: Barry the wizard casts "Time Stop", a ninth level spell. Tim Bramble casts "Deny" using a fourth level spell slot. He then takes 1d4 times 5 damage. He then rolls a 3, which means that he takes 15 necrotic damage. Yikes! 

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, a spell the creature is trying to cast of an equal or lower level to the spell slot used fails.