Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Large giant goose. The swarm can't regain hit points or gain temporary hit points.
Zelot. while the giant Goose swarm is raging, if they drop to 0 hit points, they don't die. they still roll death saves and only die when they stop their rage. the giant Goose has advantage against enemies of the God-emporer goose hydra, and is immune to the effects of the calm emotions spell.
Beaks. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 180 (60d6 + 30) piercing damage.
Break away. the swarm takes 26 points of damage. this damage cannot be negated or reduced in any way. the swarm summons a Giant Goose Zelot in an unnocupied space within 10ft of it.
Entomb. Melee Weapon Attack: Automatic hit, reach 5 ft., one target within the swarm's space. Hit: 180 (60d6 + 30) piercing damage.
One of Us (3/day). Melee Spell Attack: Automatic hit, reach 5 ft., one target within the swarm's space. Hit: the target is destroyed if it has 100 hitpoints of fewer, the swarm regains 26 hitpoints.
Rage. the swarm enters a rage. the rage lasts for 1 minute, while raging, the swarm gets +10 to attack and damage rolls, advantage on strength saves, and gains resistance to bludgeoning, slashing, and piercing damage.
Beautiful. That's all I have to say.