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Instead of requiring assists / kills to obtain stacks why not have something in which you gain bonuses over time?
UNIQUE PASSIVE: For every minute that passes, the holder gains Leviathen 1 stack. Each stack increases the holders maximum health by 1%. This effect can stack up to 20 times. Upon dying a third of the stacks are lost. At 20 stacks you gain 1% health regen.
UNIQUE ACTIVE (Requires 20 Stacks): You will absorb 50% of damage taken from all nearby friendly units for 6 seconds.