Dread Blossom Swarm
It's... a swarm of blood-sucking, flying, extraplanar plants. Not sure what else to say here.
7 plant RHD are obviously never good. Plant immunities are nice, swarm immunities a mixed bag (not being able to receive buffs isn't cool). Ability scores disappoint, with abysmal strength and charisma, average wisdom, moderate dexterity and constitution and nonexistent intelligence (note that the latter, combined with being a swarm, means that making this playable is actually kinda difficult).
Half damage from piercing and slashing weapons is nice, though being Tiny means the blossoms miss out on the far more useful immunity to weapon damage. Between Distraction and Poison Pollen anything that gets to close is very possibly going to lose actions, though it should be noted that the latter isn't friendly, affects a relatively large area, and can't be turned off. Try and find yourself a poison-immune party.
The swarm's main mode of attack is a pitiful 2d6 swarm damage, as well as a 1d6 constitution blood drain that only works against immobile targets (nauseated doesn't count, paralyzed does). Neither of those is very deadly, but that's okay as the swarm is more of a BFC character anyway.
Another notable ability is Regeneration 5, overcome only by cold and fire damage.
Finally, the melee bouquet gets three bonus feats. Ability Focus (Poison Pollen) is something most would probably pick up anyway, Lightning Reflexes isn't great but nice to get for free, and Alertness would be nearly useless if Spot and Listen weren't two of its three only class skills.
What to make of the dread blossom swarm? It's got a very useful, no-action, AoE support ability that'll cripple many a foe (at least before poison immunity becomes commonplace) but it hits allies too and can't be turned off. Furthermore, while a swarm can be granted an intelligence score with some shenanigans, it won't be easy or cheap. Then there's the usual issues of being incapable of using/wearing/wielding most items, except worse because instead of relatively humanoid animals this swarm consists of limbless flowers.
With the dread blossom swarm's only significant non-defensive ability so easily negated, and the plethora of logistical issues involved with playing one, -0 LA should be perfectly balanced.