For attack based: Make an ability based on Orb of lightnings orb ability... then make it use your Rage ability
For totally passive: Just make a trigger which creates a dummy caster whenever your unit hits HP < 60% that casts Rage on your unit... then for cooldown, you'd have to use a timed loop to store cooldown time for every unit so that it won't run when its on cooldown
Though the thing for the totally passive one is that it won't show cooldown on the UI...
or maybe you can try to do it the attack based way, but set chance to 0 so that it doesn't trigger when you attack, then on your HP trigger, just order the unit to cast the Rage ability