Here is a paste from an older pre beta thread I made:
Here is a quick overview of my vision of how the Flood could make an appearance in Halo Wars 2 multiplayer as a faction. This list is meant to be combined with my previous Halo Wars game mechanic wishlists but does not rely on them.
Infantry can transform between different forms based on the needs of the player.
each form type has its own upgrades.
many flood units produce flood spores that can infect enemy infantry and turn them into combat forms that have melee attacks. If combat forms are not killed they mutate into carrier forms (exploders) that explode near enemy units dealing damage and producing spores. if a carrier form is not activated/destroyed after a duration, it dissolves into spores. the whole process from infection to dissolution takes 40 seconds (20 second combat form 20 second carrier form) flood spores can also be built as a suicide squad consisting of 12 spores and upgrades for this squad affect all infection forms.
Flood "Vehicles" are large pureforms, most of which spawn infection form spores. The largest Flood Ground unit is the Wormform, a large immobile tentacle that deals high damage to any enemy nearby, it can relocate by burrowing but it is vulnerable for a short time after emerging. The Juggernaught is the flood answer to vehicles but needs to get in close to deal damage and relies on other units to absorb damage for it. The Flood Corpser is a mobile anti air/artillery unit.
Flood Air units are weak and rely on numbers to compete, The largest Air form is the abductor which can snare and hold enemy air units that stray too close.
Flood abilities rely on synergies for effect and are heavily based on infecting enemy infantry. While traditionally strong against light infantry, the flood faction is no slouch against vehicle heavy tactics and can adapt its swarm strategies to fight any foe. Flood units are generally weaker than their covenant and UNSC counterparts but make up for the lack of strength in raw numbers. This is achieved by lower unit cost and faster unit production.
Infection Form (flood spore): suicide anti infantry flood, infects enemy infantry into combat forms and eventually carrier forms which create more spores.
Stalker Pureform: mobile, stealthed, melee anti infantry damage, transforms into tank or ranged form
Ranged Pureform: immobile, ranged suppression anti infantry, armor mode, tansforms into stalker or tank form
Tank Pureform: armored, regeneration, melee anti armor attack, lower mobility, transforms into stalker or ranged form
Infector Pureform: spawns infection forms to attack, regenerates
Juggernaught Pureform: Mobile anti armor low range, regenerates
Corpser Pureform: armored mobile anti air artillery unit launches acid that deals damage over time reducing armor
Worm Pureform: Large burrowing tentacle, emerges dealing high damage, deals high melee damage
Flood Swarm: suicide anti air flood
Scourge Pureform: flying low health ranged forms
Bomber Pureform: slow spore spawning armored flyer
Abductor Pureform: large tentacled floater with ground and air melee attacks
Flood Rain: spawn infection forms at the target area
Rapid Mitosis: all infected infantry in the area immediately burst into infection forms
Beneath the Grounds: spawn some flood tentacles at target area
For the Gravemind: all flood units in the area move significantly faster and regenerate health for a short duration