Mutants gain unique bonuses. These are different each game.

Voidborne:
(3) Mutants execute targets they damage who are below 20% Health.
(5) And Mutants deal 40% of their damage as true damage.
(7) And Mutants deal 80% of their damage as true damage.
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Bio-Leeching: Your team gains Omnivamp (healing for a percentage of all damage dealt).
(3) 30% Omnivamp
(5) 65% Omnivamp
(7) 100% Omnivamp
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Voracious Appetite: Mutants grow stronger each time an ally dies.
(3) +18 Attack Damage and Ability Power
(5) +25 Attack Damage and Ability Power
(7) +35 Attack Damage and Ability Power
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Synaptic Web: The Abilities of Mutants cost less to cast. (Minimum 10 Mana)
(3) 20 Mana less
(5) 40 Mana less
(7) 40 Mana less, and gain 50 bonus Ability Power
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Adrenaline Rush: Mutants' attacks have a chance to trigger 2 additional attacks against their target.
(3) 40% chance
(5) 75% chance
(7) 100% chance, and gain 40 bonus Attack Damage
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Cybernetic Enhancement: Mutants with at least 1 item gain bonus stats.
(3) 450 Health and 30 Attack Damage
(5) 800 Health and 60 Attack Damage
(7) 1250 Health and 90 Attack Damage
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Metamorphosis: Mutants grow every 2 seconds, gaining bonuses that stack up to 5 times.
(3) +18 Armor and Magic Resist, +2 Attack Damage and Ability Power
(5) +25 Armor and Magic Resist, +4 Attack Damage and Ability Power
(7) +35 Armor and Magic Resist, +7 Attack Damage and Ability Power