Effect of Surprise. A creature that is surprised cannot move or take actions until after its first turn in the battle.
Moving Around Other Creatures. You can’t move through an enemy’s space unless it is two sizes larger or smaller than you. In contrast, you can move through an ally’s space, but you can’t stop there.
Instant Death. Massive damage can kill you instantly. When damage reduces you to 0 hit points and there is damage remaining, you die if the remaining damage equals or exceeds your hit point maximum.
Moving Diagonally. Entering a square diagonally costs 11⁄2 squares of movement. This rule means you must have at least 2 squares of movement left to enter your first square diagonally, and every 2 squares you move diagonally uses up 3 squares of movement.
Charge: To charge, you choose a target that is at least 10 feet away from you. You move up to half your speed to a position where that target is within your reach, and then you make a melee attack against it. After the attack, your turn ends.
The Effects of Being Stable. A stable creature doesn’t make death rolls, even though it has 0 hit points, but it does remain unconscious. The creature stops being stable, and must start making death rolls again, if it takes any damage. A stable creature that is not healed regains 1 hit point after 1d4 hours.
Longer Casting Times. Certain spells require more time to cast: minutes or even hours. If you take damage while you are casting a spell that requires more than a single action or reaction, you must succeed on a Constitution saving throw to continue casting the spell. The DC equals half the damage you just took. If you take damage from multiple sources, such as from an arrow and a dragon’s breath, you make a saving throw against each source of damage separately. If you fail the save, the spell fails, but it is not expended. If you want to cast the spell again, you must start over.