AceCalhoon’s answer is only mostly correct. While counterspell can be blocked with another counterspell, a single wizard could not counterspell a counterspell that targets a spell he is casting. Although he can take reactions, he cannot actually cast counterspell in the middle of chanting fireball.
Can you counterspell Instantaneous spells?
@JeremyECrawford Does Counterspell stop Instantaneous spells? PHB 203 says no. Internet nerds say yes.
Can you counterspell a ritual?
at any point during the casting attempt. So a ritual that requires 24 hrs to cast, can be counterspelled at the first verbalization or first somatic motion… or it can be countered as late as during the last combat round before completion. The books are not explicit on this.
What happens if your opponent tries to cast a counterspell?
If the target of your counterspell tries to cast a spell, make a Spellcraftcheck (DC 15 + the spell’s level). This check is a free action. If the check succeeds, you correctly identifythe opponent’s spell and can attempt to counter it. If the check fails, you can’t do either of these things.
How do you use a counterspell in magic?
How Counterspells Work: To use a counterspell, you must select an opponent as the target of the counterspell. You do this by choosing the ready action. In doing so, you elect to wait to complete your action until your opponent tries to cast a spell. (You may still move your speed, since ready is a standard action.)
Can you cast magic missile and Counterspell at the same time?
However, if you have a feature like action surge that gives you two actions, you can cast two 1 action spells with the 2 actions you get. In the case of the ek, you could cast magic missile, action surge, and cast magic missile again. Never DM for a group of spellcasters with Counterspell who then face enemies who also have Counterspell.
Why can I cast a reaction spell on the same turn?
Finally, if the relevant trigger occurs, you could cast a reaction spell (e.g. you would get hit by an opportunity attack so you cast shield, or you get damaged by the attack so you cast hellish rebuke, or an enemy tries to counterspell your action spell so you cast counterspell on it).
