DMG Action Options
These are new action options for combat, added individually in alphabetical order taken from PG. 271 of the Dungeon Master's Guide.
Climbing onto a Bigger Creature
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 to get into position, the smaller creature uses its action or bonus action to make a Strength (Athletics) or Dexterity (Acrobatics) check contested by the target's Strength (Athletics) or Dexterity (Acrobatics) check. If it wins the contest, the smaller creature successfully moves into the creature's space.
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 on the larger creature's space, treating the space as difficult terrain. The larger creature's ability to attack the smaller creature dep-ends on the smaller creature's location, and is left to the DM's discretion. The larger creature can dislodge the smaller creature as an action, bonus action, or its full movement (only one option on their turn)---knocking it off, scraping against a wall, grabbing and throwing it, rolling around, bucking up and down, or even falling prone---by making a Strength (Athletics) check contested by the smaller creature's Strength (Athletics) check or Dexterity (Acrobatics) check. The smaller creature chooses which ability to use.
Disarm
A creature can use a weapon attack to knock a weapon or another item from the 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 at its feet or the item gets flung up to 20 feet away in a direction of the attacker's choice (attacker's choice of drop or fling).
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 smaller creature, or disadvantage if it is smaller.
Overrun
When a creature moves through a hostile creature's space, they can try to force its way by using an action or bonus action.
You make 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.
Shove Aside
Whenever 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 to do so, but moves the target 5 feet to a different space within its reach.
Tumble
A creature can try to tumble through a hostile creature's space. As an action or bonus action, the tumbler makes a Dexterity (Acrobatics) check contested by the hostile creature's Dexterity (Acrobatics) check. If the tumbler wins this contest, it can move through the hostile creature's space once this turn.
