Proof of stake replaces the cost of electricity with the risk of losing capital. Slashing is what turns a deposit into a genuine cost, and without it the deposit would secure nothing.
Security requires a penalty, not just a deposit
In proof of work, attacking a chain is expensive because the electricity is spent whether or not the attack succeeds.
A staked deposit costs nothing to hold, so it deters nothing by itself. The deterrent comes from the deposit being destroyable.
Slashing supplies that property by removing part of the stake belonging to a validator that breaks specific rules.
Offences must be provable from the chain
Slashing is automatic, so the conditions have to be things any node can verify without judgement.
Signing two different blocks for the same slot is one, since both signatures exist and contradict each other.
Signing contradictory attestations is another, and both amount to a validator asserting two versions of history at once.
Anyone can submit the evidence, and the protocol applies the penalty on receipt. No committee reviews the case and no intent is considered, because neither could be evaluated by software.
The penalty scales with how many are involved
A single validator equivocating is usually a misconfiguration, such as running the same keys on two machines, and attracts a modest penalty.
Many validators committing the same offence simultaneously looks like a coordinated attack, and the penalty rises steeply with the number involved.
This makes an attack requiring many validators disproportionately expensive while treating an individual accident as an accident.
The correlation is measured over a window rather than instantaneously, so an operator running a large number of validators has a strong reason to keep their infrastructure independent of itself.
Being offline is treated differently
A validator that fails to participate is not slashed. It loses the rewards it would have earned and suffers a small ongoing reduction.
The distinction matters because downtime harms the network mildly while equivocation attacks its consistency directly.
Inactivity penalties do escalate if a large share of validators is offline at once, which pushes the network back towards finalisation.
What it means for anyone staking
Delegated and pooled staking passes slashing risk to the depositor, since the penalty falls on the stake regardless of who operates the hardware.
The practical risks are operational: duplicated keys, failed migrations and unreliable infrastructure cause most incidents rather than deliberate misbehaviour.
Understanding who bears that risk under a given arrangement matters more than the headline yield, because the two are describing different sides of the same position.