This optional rule makes it easier for a creature to be felled by massive damage.

When a creature takes damage from a single source equal to or greater than half its hit point maximum, it must succeed on a DC 15 Constitution saving throw or suffer a random effect determined by a roll on the System Shock table.

For example, a creature that has a hit point maximum of 30 must make that Constitution save if it takes 15 damage or more from a single source.

System Shock

d8	Effect
1-2	The creature drops unconscious but is stable.
3-4	The creature is stunned until the end of its next turn.
5-6	The creature can't take reactions and has disadvantage on attack rolls and ability checks until the end of its next turn.
7-8	The creature can't take reactions until the end of its next turn.

A creature can try to tumble through a hostile creature's space, ducking and weaving past the opponent. As an action or a bonus action, the tumbler makes a Dexterity (Acrobatics) check contested by the hostile creature's Dexterity (Acrobatics) check. If the tumbler wins the contest, it can move through the hostile creature's space once this turn.  The hostile creature's space is considered difficult terrain.

A creature can use a weapon attack to knock a weapon or another item from a target's grasp. The attacker makes an attack roll contested by the target's Strength (Athletics) check or Dexterity (Acrobatics) check. If the attacker wins the contest, the attack causes no damage or other ill effect, but the defender drops the item.

The attacker has disadvantage on its attack roll if the target is holding the item with two or more hands. The target has advantage on its ability check if it is larger than the attacking creature, or disadvantage if it is smaller.