You attempt to interrupt a creature in the process of casting a spell. The creature makes a saving throw of their spell casting ability. The DC is equal to your spell save DC with an additional +1 bonus for every spell slot above 3 level. The creature may use its Bonus Action after the save but before the outcome is known to use an additional spell slot to add a bonus to their saving throw. The bonus will be equal to +1 per level of spell slot used.
Any allied creature within 30 feet of the counter-spelled spell’s point of impact that is aware of the contest may use a reaction to add one of their spell slots to overcome the counterspell (+1 per spell slot level). Bonuses from all sources are aggregated for the purposes of overcoming the counterspell. All spell slots activated during this process are used regardless of the outcome of the counterspell.
Counterspell cannot be counterspelled.
* - which you take when you see a creature within 60 feet of yourself casting a spell with Verbal, Somatic, or Material components
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