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, 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 the DM's 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.
Using the Attack action, you can make a special melee attack to shove a creature, either to force it prone or push it away from you. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them.
The target must be no more than one size larger than you and must be within your reach.
Instead of making an attack roll, you make a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use).
If you win the contest, you either force the target prone or push it 5 feet away from you.
You can also try to move it 5 feet to a different space within your reach, which gives you disadvantage at your Strength (Athletics) check.
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.
The grapple ends if the grappler is incapacitated. The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is forcefully hurled away by a sphere talent or ability.
Escaping a Grapple: A grappled creature can use its action to escape. To do so, it must succeed on a Strength (Athletics) or Dexterity (Acrobatics) check contested by your Strength (Athletics) check.
Moving a Grappled Creature: When you move, you can drag or carry the grappled creature with you, but your speed is halved, unless the creature is two or more sizes smaller than you.
When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple. If you’re able to make multiple attacks with the Attack action, this attack replaces one of them.
The target of your grapple must be no more than one size larger than you and must be within your reach.
Using at least one free hand, you try to seize the target by making a grapple check instead of an attack roll: a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). If you succeed, you subject the target to the grappled condition. You can release the target whenever you like (no action required).
A grappled creature’s speed becomes 0, and it can’t benefit from any bonus to its speed.