Harmony’s whitepaper states the following:

• If a leader fails to finish the consensus process and triggered the view change process, P_vote staked tokens will be slashed
• If validators are proven to sign a dishonest block, all of their stake under the same shard will be slashed
• Any validator can submit a transaction to prove the misbehavior of another validator and if verified, the slashed token will be rewarded to the prover

Different Protocol’s Slashing Mechanism

Ethereum

• A validator acts as a “whistleblower” — a validator that monitors for slashable events
• Whistleblower must create and disseminate a specific message outlining the infraction, which a propose then adds to a block
• To compensate the block proposers for slashing the bad validator, they are rewarded with the offender’s effective balance divided by 512 (e.g. roughly 0.06 ETH if the offender’s balance is 32 ETH)
• Whistleblower themselves are not rewarded; the reward is purposefully small as these acts are meant to be a selfless gesture, not a means to profit

The above is pre-Bellatrix; the below contains information from Bellatrix upgrade (https://eth2book.info/bellatrix/part2/incentives/slashing/)

• Slashing triggered by the evidence of the offense being included in a beacon chain block
• Initial Penalty
  • Offender immediately has 1/32 of its effective balance deducted from its actual balance (maximum of 1 ETH due to the cap on effective balance)
  • Introduced to make it costly for validators to self-slash for any reason
  • Validator is queued for an exit with withdrawability epoch set to around 36 days (~8192 epochs)
• Correlation Penalty
  • Occurs at the halfway point of its withdrawability period (18 days after being slashed)
  • Based on the total amount of stake slashed during the 18 days before and after the validator was slashed
  • The idea is to scale the punishment according to the events that happened during the time
  • Calculated as follows:
    • Compute the sum of the effective balances of all validators slashed in the 36 days span
    • Multiply the sum by PROPORTIONAL_SLASHING_MULTIPLIER_BELLATRIX but cap the result at total_balance, the total active balance of all

Harmony’s Implementation

When a new node is started through New(...) (node.go) function, the following goroutine is started:

If a doubleSign record is received in the SlashChan, it is processed by the node (TODO: When and where is the doubleSign message formed in the first place?). Depending on whether the node is a running in beacon chain or not, the record will either be added to the pending slashing candidates or broadcasted respectively.