Concentration

[Not a new rule, just often forgotten about]

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.

The following factors can break concentration:

• Casting another spell that requires concentration You lose concentration on a spell if you cast another spell that requires concentration. You can't concentrate on two spells at once.
• Taking damage. Whenever you take damage while you are concentrating on a spell, you must make a Constitution saving throw to maintain your concentration. The DC equals 10 or half the damage you take, whichever number is higher. Constitution saving throw DC 10 or half damage (higher of the two) If you take damage from multiple sources, such as an arrow and a dragon's breath, you make a separate saving throw for each source of damage.
• Being incapacitated or killed You lose concentration on a spell if you are incapacitated or if you die.
• At will You can end concentration at any time (no action required).