From Halopedia, the Halo wiki
Airborne, squid-like body with numerous tendrils
Following the establishment of a self-sufficient Flood hive, the infestation begins to spawn more specialised kinds of infection form suited for niche roles and environments that are beyond the purview of the standard pod infectors. The seeders serve as lighter-than-air airborne Flood forms, capable of "swimming" through the sky to hunt airborne prey and coordinate strikes against ground forces. They typically tend to gather into large swarms capable of defending the Flood's territory by means of blocking access of the skies to aircraft - seeder infectors can infect aircraft and convert them to Flood use, but are also capable of spitting bone fragments of their victims at enemy targets.
Seeders were used by the Flood en masse during the Outbreak on Installation 00, in mid-2559. During the conflict, they saw action against the forces of the United Nations Space Command and Banished.
Halo Wars 2
Seeders are the aerial equivalent of the land-bound Flood infester form as they are both primarily meant to infect enemy vehicles. Seeders, however, differ slightly with the abovementioned ability to also attack ground infantry with bone fragments. Nevertheless, Seeders are incredibly vulnerable and must rely on numbers and speed to remain effective.
Once the parasite can establish a hive it can begin to produce massive numbers of infection forms that spread out in search of new sentients to infect, and non-sapient animals on which to feed. Both ground and airborne infection forms are used as living weapons, hurling themselves at the enemy to overwhelm defenses and assimilate the unprepared.
Seeders are lighter-than-air Flood units that swim through the sky and attack aerial units. If a Seeder gets the killing blow, the crew are overwhelmed and the aircraft becomes Infected. Seeders only infect aircraft, but will "spit" shards of bone at ground targets.
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