Counterspell

Counterspell is now a saving throw, and big monsters are hard to stop

At the table. Counterspell is a Reaction you take when you see a creature within 60 feet casting a spell with Verbal, Somatic or Material components. The caster makes a Constitution saving throw against your spell save DC. If it fails, the spell does nothing, and the action spent on it is lost.

What changed from 2014. The SRD 5.1 version stopped any spell of 3rd level or lower outright, and for higher spells you rolled an ability check against 10 + the spell's level. Casting it with a higher slot raised the level it stopped automatically. The 2024 version drops all of that: every spell, of any level, comes down to the target's Constitution save. It also can't be upcast. And if a spell cast with a slot is countered, the caster keeps the slot.

How often does it work? We counted the d20 faces for two casters: a level 5 wizard (DC 15) and a level 17 wizard (DC 19). Constitution saves are from the SRD 5.2.1 stat blocks.

The dragon and the lich also have Legendary Resistance: a failed save can become a success a few times a day. So against them, Counterspell mostly burns Legendary Resistance.

The choice that matters is what you let through. You get one Reaction per round. If you spend it on a cantrip, you can't Shield the attack that follows.

The common mistake is holding it all fight for the perfect spell. Stopping a spell that would end the fight is worth more than a turn's damage; stopping the mage's second-best spell usually isn't.

The human angle. Being countered feels worse than missing an attack. Kahneman and Tversky showed people weigh a loss more heavily than a gain of the same size. The 2024 rule that the countered caster keeps their slot takes some of the sting out of it, whichever side of the table you're on.

Source: Kahneman, D., & Tversky, A. (1979). Prospect theory: An analysis of decision under risk. Econometrica, 47(2), 263–291. doi:10.2307/1914185