Actions

Multiattack. The horde makes 3/4 attacks, or two assault attacks.

Yari. Melee weapon attack, 10ft, one creature. To hit: 1d20+8 / 1d20+10 , damage: 2d10+5 / 4d10+5 .

Stampede. When the horde uses the Dash action, it may choose to Stampede instead. If the horde enters the space of one or several individual creatures that are no more than one size larger than an individual member of the horde, they must succeed on a DC 17 Strength saving throw or be knocked prone.

Protect. A horde can allow friendly creatures to enter its occupied space and protect them from enemy attacks. A horde can protect up to three creatures of the same size as the horde's individual creatures. Protected creatures can Disengage at any time to cancel this condition. The Protected creatures benefit from three-quarters cover (+5) against ranged attacks unless the horde is Bloodied.

Surround. If the horde occupies the space of one or several individual creatures that are no more than one size larger than an individual member of the horde, it can use its action to surround them. The creatures must succeed on a Dexterity saving throw or become Surrounded. The DC of this saving throw is equal to 8 + the horde's proficiency bonus + the horde's Dexterity modifier.
Surrounded creatures can be targeted by Assault Attack. They cannot use Dash or Disengage while surrounded, and provoke an opportunity attack if they use their Move action. If a creature that is Surrounded deals damage to the horde, it can attempt a Dexterity saving throw against the horde's DC (17) at the end of the turn. On a successful save, it is no longer Surrounded and can use its movement to leave the space of the horde without provoking an opportunity attack.

Spread. The Horde can use its action to spread thin across a larger unoccupied area. The Horde needs a 40 feet by 40 feet area on a map to spread successfully. While spread, the Horde can make only 2 attacks (1 if bloodied) as part of its multiattack, and cannot be targeted by Volley attacks or targeted by spells with a range of 30 feet or less or that targets a large number of creatures (10 creatures or more).
While the Horde is spread, all creatures that occupy its space lose the Surrounded or Protected conditions if they had them.

Collide. If the horde occupies some of the space of another horde and both hordes are not already Surrounded, it can use its action to collide with it. Both hordes suffer the Surrounded condition from each other. Both hordes make an opposed Strength check, adding their proficiency bonus. The horde that wins the contest can immediately make one Surround Attack against the other horde.

Assault Attack. Hordes that use surround tactics can use their action to make a single Assault Attack that targets all creatures that occupy its space and suffer the Surrounded condition. Assault attacks are listed in the horde's statistics.
When the horde makes an Assault Attack, all targets must make a DC 17 Dexterity saving throw. On a failed save, each target takes damage equal to 4 times an individual creature's weapon dice, or 2 times an individual creature's weapon dice if the horde is Bloodied. On a successful save, it takes half as much damage.

Reactions

Reaching Strike. When a creature enters within melee range of the horde, the horde can make an attack of opportunity on the creature.