Undead Swarm. There are 100 Zombie Swarms in the Tide. For every 100 damage dealt to the Tide, 1 Zombie Swarm dies, reducing the effectiveness of the Tide. For every Zombie Swarm killed, the Tide takes a -30 to its attack, and deals 40 less damage on a hit. 

Lake of Targets. If the Tide is hit with an area of effect, it takes extra damage. Multiply the damage dealt by every 5x5 area hit after the first.

To the Last.  If the Tide drops below half health, it is considered Huge. At 1/4 it is considered Large. If the Tide is reduced to 5 Zombies, it becomes a Zombie Swarm.

Pyramid. The Tide can use half its Movement to create a Pyramid up to 40ft high. Zombie Swarms may be spawned from the top of the Pyramid. If 100 Damage is dealt to the Pyramid, it collapses.

Consume. Melee Weapon Attack: +150 to hit, reach 5ft., all adjacent targets. Hit: 2000 bludgeoning damage.

Break Off. Spawn a Zombie Swarm adjacent to the Tide at the cost of 100 of the Tide's HP.

Shifting Tide. If a Zombie Swarm is adjacent to the Tide, it can be added to the Tide's numbers. The Zombie Swarm is destroyed, and its current HP is added to the Tide's. If this would heal the Tide beyond its maximum HP, the excess healing is converted to Temp HP instead. 

Break Off. Spawn a Zombie Swarm adjacent to the Tide at the cost of 100 of the Tide's HP. (1 Legendary Action)

Shifting Tide. If a Zombie Swarm is adjacent to the Tide, it can be added to the Tide's numbers. The Zombie Swarm is destroyed, and its current HP is added to the Tide's. If this would heal the Tide beyond its maximum HP, the excess healing is converted to Temp HP instead. (1 Legendary Action)

Dinner Bell. The Tide moves 5ft closer to the nearest target. (1 Legendary Action)