Ability damage
Some attacks and effects damage an ability score instead of, or as well as, hit points.
Bloodied
A creature is bloodied when it has half its hit point maximum or fewer hit points. Some features and effects key off this state.
Cover
Obstacles between an attacker and a target can give the target cover. Cover only counts against attacks and effects that come from the opposite side of the obstacle.
Critical hits
On a d20, a natural 1 always fails and a natural 20 always succeeds. This applies to ability checks and saving throws as well as attack rolls.
Critical hit and fail effects
When an attack roll is a natural 20 or a natural 1, the DM rolls for an extra effect. Results can include bonus damage, conditions, ability damage or something stranger.
Flanking
Flanking gives advantage on melee attacks.
Identifying spells in combat
A character automatically recognizes any spell cast in front of them that appears on their own class's spell list.
Potions
Quickened Spell
A sorcerer who uses Quickened Spell can still use their action to cast another leveled spell on the same turn. The usual limit of one leveled spell per turn doesn't apply to this combination.
Spell scroll mishap
If a character fails to cast a spell from a scroll, they make a DC 10 Intelligence saving throw. On a failure, the DM rolls on the Scroll Mishap table in the Dungeon Master's Guide.