Tankbuster is a Tier 3 Spirit Item which grants Bonus Health to the purchasing hero. It was previously named Improved Burst.
It has a passive which causes additional Spirit Damage to enemies who receive a single instance of ability spirit damage above a certain threshold from the purchasing hero.
It can be purchased from the Shop for 3,200 (its Component, Mystic Burst, makes up 800 of the cost).
Tankbuster only goes on cooldown when it affects enemy heroes. It will not go on cooldown when you hit a Trooper.
Some forms of Spirit Amp applied by items count as separate damage instances and thus don't contribute to the single hit such as Escalating Exposure.
When the target is hit with spirit damage, the damage value which is compared against the triggering threshold is the base damage value prior to any increases or decreases from damage amp, damage reduction, spirit resistance, and spirit resistance reduction.
Damage amp and spirit resist reduction will not help you in achieving the damage threshold to trigger the passive.
Damage reduction and the target's spirit resistance will not prevent the passive from triggering when it otherwise would.
Interaction with spirit shields: The triggering of the additional damage is entirely unaffected by the target's barriers (or lack thereof).
The additional damage, and the ability which triggered the additional damage, are applied to the target as separate instances of damage. The additional damage from Tankbuster is dealt after the original spell damage. This has implications for abilities or effects that rely on single instances of damage, for example:
Vindicta uses assassinate on an enemy Warden. The baseline Assassinate on its own is not enough damage to kill the Warden, but the increased damage (due to the additional damage from triggering Tankbuster) is enough to secure the kill. Since the additional damage from triggering Improved Burst is applied after the Assassinate, it is technically the item's damage which secures the kill, not the Assassinate. Thus, Vindicta will not receive bonus souls from Assassinate's passive.
3,200
14s
bonus spirit damage, causing abilities dealing more than 165 damage to deal additional damage. Ignores Spirit Resistance.
Mystic Burst
Vindicta uses assassinate on an enemy 
Warden. The baseline Assassinate on its own is not enough damage to kill the 
