These are rules from the DMG, posted here so my players can reference them.
CLIMB ONTO A BIGGER CREATURE
If one creature wants to jump onto another creature, it can do so by grappling. A Small or Medium creature has little chance of making a successful grapple against a Huge or Gargantuan creature, however, unless magic has granted the grappler supernatural might.
As an alternative, a suitably large opponent can be treated as terrain for the purpose of jumping onto its back or clinging to a limb. After making any ability checks necessary to get into position and onto the larger creature, the smaller creature uses its action to make a Strength (Athletics) or Dexterity (Acrobatics) check contested by the target's Dexterity (Acrobatics) check. If it wins the contest, the smaller creature successfully moves into the target creature's space and clings to its body. While in the target's space, the smaller creature moves with the target and has advantage on attack rolls against it.
The smaller creature can move around within the larger creature's space, treating the space as difficult terrain. The larger creature's ability to attack the smaller creature depends on the smaller creature's location, and is left to your discretion. The larger creature can dislodge the smaller creature as an action- knocking it off, scraping it against a wall, or grabbing and throwing it- by making a Strength (Athletics) check contested by the smaller creature's Strength (Athletics) or Dexterity (Acrobatics) check. The smaller creature chooses which ability to use.
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.
This option makes it easier for melee combatants to harry each other with opportunity attacks.
When a creature makes a melee attack, it can also mark its target. Until the end of the attacker's next turn, any opportunity attack it makes against the marked target has advantage. The opportunity attack doesn't expend the attacker's reaction, but the attacker can't make the attack if anything, such as the incapacitated condition or the shocking grasp spell, is preventing it from taking reactions. The attacker is limited to one opportunity attack per turn.
When a creature tries to move through a hostile creature's space, the mover can try to force its way through by overrunning the hostile creature. As an action or a bonus action, the mover makes a Strength (Athletics) check contested by the hostile creature's Strength (Athletics) check. The creature attempting the overrun has advantage on this check if it is larger than the hostile creature, or disadvantage if it is smaller. If the mover wins the contest, it can move through the hostile creature's space once this turn.
With this option, a creature uses the special shove attack from the Player's Handbook to force a target to the side, rather than away. The attacker has disadvantage on its Strength (Athletics) check when it does so. If that check is successful, the attacker moves the target 5 feet to a different space within its reach.
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.
If the attack roll falls within a range low enough to miss the target but high enough to strike the target if there had been no cover, the object used for cover is struck. If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.
CLEAVING THROUGH CREATURES
When a melee attack reduces an undamaged creature to 0 hit points, any excess damage from that attack might carry over to another creature nearby. The attacker targets another creature within reach and, if the original attack roll can hit it, applies any remaining damage to it. If that creature was undamaged and is likewise reduced to 0 hit points, repeat this process, carrying over the remaining damage until there are no valid targets, or until the damage carried over fails to reduce an undamaged creature to 0 hit points.
Damage normally leaves no lingering effects. A creature might sustain a lingering injury under the following circumstances:
When it takes a confirmed critical hit. (Not mooks)
When it drops to 0 hit points but isn't killed outright. (Not mooks)
When it fails a death saving throw by 5 or more
Mooks neither suffer nor inflict lingering injuries, except indirectly through a failed death saving throw. A mook is any unnamed creature with a challenge less than the character's level.
To determine the nature of the injury, roll on the Lingering Injuries table. This table assumes a typical humanoid physiology, but you can adapt the results for creatures with different body types.
1 Lose an Eye. You have disadvantage on Wisdom (Perception) checks that rely on sight and on ranged attack rolls. Magic such as the regenerate spell can restore the lost eye. If you have no eyes left after sustaining this injury, you're blinded.
2 Lose an Arm or a Hand. You can no longer hold anything with two hands, and you can hold only a single object at a time. Magic such as the regenerate spell can restore the lost appendage.
3 Lose a Foot or Leg. Your speed on foot is halved, and you must use a cane or crutch to move unless you have a peg leg or other prosthesis. You fall prone after using the Dash action. You have disadvantage on Dexterity checks made to balance. Magic such as the regenerate spell can restore the lost appendage.
4 Limp. Your speed on foot is reduced by 5 feet. You must make a DC 10 Dexterity saving throw after using the Dash action. If you fail the save, you fall prone. Magical healing removes the limp.
5-7 Internal Injury. Whenever you attempt an action in combat, you must make a DC 15 Constitution saving throw. On a failed save, you lose your action and can't use reactions until the start of your next turn. The injury heals if you receive magical healing or if you spend ten days doing nothing but resting.
8-10 Broken Ribs. This has the same effect as Internal Injury above, except that the save DC is 10.
11-13 Horrible Scar. You are disfigured to the extent that the wound can't be easily concealed . You have disadvantage on Charisma (Persuasion) checks and advantage on Charisma (Intimidation) checks. Magical healing of 6th level or higher, such as heal and regenerate, removes the scar.
14-16 Festering Wound. Your hit point maximum is reduced by 1 every 24 hours the wound persists. If your hit point maximum drops to 0, you die. The wound heals if you receive magical healing. Alternatively, someone can tend to the wound and make a DC 15 Wisdom (Medicine) check once every 24 hours. After ten successes, the wound heals.
17-20 Minor Scar. The scar doesn't have any adverse effect. Magical healing of 6th level or higher, such as heal and regenerate, removes the scar.
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.
1 The creature drops to 0 hit points.
2-3 The creature drops to 0 hit points but is stable.
4-5 The creature is stunned until the end of its next turn.
6-7 The creature can't take reactions and has disadvantage on attack rolls and ability checks until
the end of its next turn.
8-10 The creature can't take reactions until the end of its next turn.