One of the big advantages of creating your own hive fleet list is that you can also include one-off 'mutant' creatures in your broods which exhibit unusual capabilities or weaponry. Each brood may include mutants. However, the more diverse the species you employ in your hive fleet list, the less opportunity there is to create mutants.
To represent this, the total number of models in a brood must equal or exceed the number of different species in your hive fleet list before one of them may be a[sic?] upgraded to become a mutant. If the creatures in the brood have multiple Wounds, count the number of Wounds rather than number of models.
For example: Hive Fleet Arachnia has eight species, therefore a brood must comprise eight models (or Wounds) or more in order to include a mutant. A brood with 16 models or more could include two mutants, and so forth.
Mutation Types
Mutant creatures may be given one (and only one) mutation from the following list at the points cost indicated. A brood including multiple mutants can include different types. Mutations must be shown by an appropriate conversion.
Hive Node: The mutant is exceptionally receptive to the imperatives of the hive mind, such that it can sense commands even if Synapse creatures are not nearby. This increases its Leadership characteristic to 10. This benefit extends to the entire brood the mutant is part of as long as the mutant is still alive. Cost: 10 points
Errata — Chapter Approved: Codex: Tyranids Q&A (v2.0): Multiple single-model broods of the same species (eg three separate Lictor broods) count as individuals for mutation purposes, not as one combined brood — they're treated somewhat like specialist mutations already, in effect. Separate broods (such as a Hive Tyrant and its Tyrant Guard) are likewise never combined for working out how many models/Wounds are needed to include a mutant. The bonus Toughness a mutant gains from Exceptional Size does count towards Instant Death. In the Shooting phase, a mutant in a squad is treated like a heavy weapon trooper — it isn't a separately-targetable model that can be sniped out with a single hit, it's simply the last model removed as the unit takes casualties (or removed along with everyone else under a template).
Weapon-beast: The mutant replaces its normal bio-weapon with any of the bio-weapons listed at no extra cost. It is not limited to just the weapons in the army list entry or on its biomorph table. If a creature which normally carries more than one bio-weapon is mutated into a weapon-beast only one of its original bio-weapons is replaced.
Cost: Close combat bio-weapon: 2 points per point of Strength the mutant has.
Cost: Ranged bio-weapon: 10 points + 3 points per point of Strength the mutant has.
Exceptional Size: The mutant is a giant amongst its kind, the dominant beast of the brood. The mutant's Strength and Toughness are increased by +1 each to a maximum of 10.
Cost: The creature's Strength multiplied by its Toughness (after the additional +1s have been added).
Acid Blood: The mutant has a powerful and unpleasant bio-acid instead of normal ichor. If the mutant is killed in close combat it will inflict a wound on all non-Tyranid models in base contact automatically. Armour saves apply as normal. Vehicles (including Dreadnoughts) suffer a hit with an armour penetration value equal to the creature's Strength + D6.
Cost: 10 points